Add os-availability-zone extension

* Query /os-availability-zone to get an object representing the configured
  availability zones and their state
* This implements a subset of Nova's os-availability-zone extension

Fixes bug 1195461

Change-Id: Ic0a8eb5a82ca0a4eed3b1e1cd6cf3a4665589307
Merge "Add os-availability-zone extension"