format-disks used to run with non-strict bash semantics, but this changed in
1.2 as we now merge it into the GCE script, so pipefail and errexit are both
set.

However, the way we list the ephemeral disks, by piping to grep, would cause an
exit code of 2 if there were no ephemeral disks.

Tolerate failure here by add `|| true`.  The metadata service call is unlikely
to fail, so we continue to ignore that possibility.

We rename it to EPHEMERAL_BLOCK_DEVICE_MAPPINGS, and we also change the value
so that it starts with a `,`, instead of always inserting a comma before it.
In this way the value can be empty.

Also, if the user sets the (currently experimental) KUBE_AWS_STORAGE
environment variable to be "ebs", then we will not mount any instance storage
which will cause the machines to use EBS storage instead.