Fix bug in os-availability-zone extension.

Fixes bug 1167956.
The call to get the list of availability zones (GET V2/os-availability-zone)
doesn't hide internal availability zone if all services in that zone are
disabled.

Change-Id: I2c49ad68c2ee5ddea0536dfa770f649fbb392b63
(cherry picked from commit 7172fdc2bf4061039fe34cdb97e442c073a7570a)
Merge "Fix bug in os-availability-zone extension." into stable/grizzly