Add availability-zone-list command

This is client-side support for the os-availability-zone extension added in
https://review.openstack.org/34813.  As /os-availability-zone/detail is not
yet implemented, adjustments were made to accomodate the lack of hosts
in the returned zone list.

Added for both v1 and v2, basically a copy of the equivalent novaclient command.

Change-Id: Iae806a2b5ea3a2d3c984a138d9c27e169160766e
Merge "Add availability-zone-list command"