fix(entitytags): Reconciliation API for Entity Tags (#2625)
This PR introduces a new administrative api supporting reconcilation of
entity tags against the current cached set of server groups.
Entity Tags that reference a non-existent server group will be removed
from elastic search.
A subsequent effort will provide a means to perform reconciliation
against `front50`.
The API is exposed directly on `clouddriver`:
```
curl -X POST localhost:8101/admin/tags/reconcile?cloudProvider=aws&account=test®ion=us-west-2
```