Revert "Disable volume AZ select for snapshot source"

This reverts commit b4078e2dd4bf6c1d42b51ebbbd39336069ab8367.
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
(cherry picked from commit 210d58de0e28d562ab6b315ed33aec9cdf75e515)

Conflicts:
	openstack_dashboard/dashboards/project/volumes/forms.py
Fix creating volumes from a snapshot

AZ is not required, so make sure the form validation doesn't check for
it.

Change-Id: I222c00a4aacd048943aae6ed50cca37bc878a23c
Categorize images list in volume create form

Change-Id: I485fe6d350225154f80dd47e1f84b7b97dacc61c