Hide AZ when creating Volume from snapshot

Volume created from snapshot must match the AZ of the snapshot.
It doesn't make sense to allow the user to select the AZ as the
only valid values are Any AZ or AZ of the snapshot - which the user
still need to figure out.

Change-Id: Id3fb03fc2bc152ae0af72b939f63d9d403324fe6
Closes-Bug: #1257845
Merge "Hide AZ when creating Volume from snapshot"