…kerd#3873)

**Subject**
Fixes bug where override of Docker registry was not being applied to debug containers (linkerd#3851)

**Problem**
Overrides for Docker registry are not being applied to debug containers and provide no means to correct the image.

**Solution**
This update expands the `data.proxy` configuration section within the Linkerd `ConfigMap` to maintain the overridden image name for debug containers at _install_-time similar to handling of the `proxy` and `proxyInit` images.

This change also enables the further override option of the registry for debug containers at _inject_-time given utilization of the `--registry` CLI option.

**Validation**
Several new unit tests have been created to confirm functionality.  In addition, the following workflows were run through:

### Standard Workflow with Custom Registry
This workflow installs Linkerd control plane based upon a custom registry, then injecting the debug sidecar into a service.

* Start with a k8s instance having no Linkerd installation
* Build all images locally using `bin/docker-build`
* Create custom tags (using same version) for generated images, e.g. `docker tag gcr.io/linkerd-io/debug:git-a4ebecb6 javaducky.com/linkerd-io/debug:git-a4ebecb6`
* Install Linkerd with registry override `bin/linkerd install --registry=javaducky.com/linkerd-io | kubectl apply -f -`
* Once Linkerd has been fully initialized, you should be able to confirm that the `linkerd-config` ConfigMap now contains the debug image name, pull policy, and version within the `data.proxy` section
* Request injection of the debug image into an available container.  I used the Emojivoto voting service as described in https://linkerd.io/2/tasks/using-the-debug-container/ as `kubectl -n emojivoto get deploy/voting -o yaml | bin/linkerd inject --enable-debug-sidecar - | kubectl apply -f -`
* Once the deployment creates a new pod for the service, inspection should show that the container now includes the "linkerd-debug" container name based on the applicable override image seen previously within the ConfigMap
* Debugging can also be verified by viewing debug container logs as `kubectl -n emojivoto logs deploy/voting linkerd-debug -f`
* Modifying the `config.linkerd.io/enable-debug-sidecar` annotation, setting to “false”, should show that the pod will be recreated no longer running the debug container.

### Overriding the Custom Registry Override at Injection
This builds upon the “Standard Workflow with Custom Registry” by overriding the Docker registry utilized for the debug container at the time of injection.

* “Clean” the Emojivoto voting service by removing any Linkerd annotations from the deployment
* Request injection similar to before, except provide the `--registry` option as in `kubectl -n emojivoto get deploy/voting -o yaml | bin/linkerd inject --enable-debug-sidecar --registry=gcr.io/linkerd-io - | kubectl apply -f -`
* Inspection of the deployment config should now show the override annotation for `config.linkerd.io/debug-image` having the debug container from the new registry.  Viewing the running pod should show that the `linkerd-debug` container was injected and running the correct image.  Of note, the proxy and proxy-init images are still running the “original” override images.
* As before, modifying the `config.linkerd.io/enable-debug-sidecar` annotation setting to “false”, should show that the pod will be recreated no longer running the debug container.

### Standard Workflow with Default Registry
This workflow is the typical workflow which utilizes the standard Linkerd image registry.

* Uninstall the Linkerd control plane using `bin/linkerd install --ignore-cluster | kubectl delete -f -` as described at https://linkerd.io/2/tasks/uninstall/
* Clean the Emojivoto environment using `curl -sL https://run.linkerd.io/emojivoto.yml | kubectl delete -f -` then reinstall using `curl -sL https://run.linkerd.io/emojivoto.yml | kubectl apply -f -`
* Perform standard Linkerd installation as `bin/linkerd install | kubectl apply -f -`
* Once Linkerd has been fully initialized, you should be able to confirm that the `linkerd-config` ConfigMap references the default debug image of `gcr.io/linkerd-io/debug` within the `data.proxy` section
* Request injection of the debug image into an available container as `kubectl -n emojivoto get deploy/voting -o yaml | bin/linkerd inject --enable-debug-sidecar - | kubectl apply -f -`
* Debugging can also be verified by viewing debug container logs as `kubectl -n emojivoto logs deploy/voting linkerd-debug -f`
* Modifying the `config.linkerd.io/enable-debug-sidecar` annotation, setting to “false”, should show that the pod will be recreated no longer running the debug container.

### Overriding the Default Registry at Injection
This workflow builds upon the “Standard Workflow with Default Registry” by overriding the Docker registry utilized for the debug container at the time of injection.

* “Clean” the Emojivoto voting service by removing any Linkerd annotations from the deployment
* Request injection similar to before, except provide the `--registry` option as in `kubectl -n emojivoto get deploy/voting -o yaml | bin/linkerd inject --enable-debug-sidecar --registry=javaducky.com/linkerd-io - | kubectl apply -f -`
* Inspection of the deployment config should now show the override annotation for `config.linkerd.io/debug-image` having the debug container from the new registry.  Viewing the running pod should show that the `linkerd-debug` container was injected and running the correct image.  Of note, the proxy and proxy-init images are still running the “original” override images.
* As before, modifying the `config.linkerd.io/enable-debug-sidecar` annotation setting to “false”, should show that the pod will be recreated no longer running the debug container.

Fixes issue linkerd#3851 

Signed-off-by: Paul Balogh javaducky@gmail.com