Fix multi availability zone issue part 1

Current aggregate APIs allow user to add multi
AZs to a host, but Openstack support single AZ
of a host. When booting an instance, nova uses
the first AZ assigned to a host,even if user
assigned multi AZs to the host and user chose
the second AZ to boot an instance. So multi AZs
aren't supported by current Openstack design,
we should fix aggregate APIs to align the design
. This patch is for limit user to add multi AZs
using aggregate-add-host API.

The part 2 limits user to add multi AZs using aggregate
update and aggregate metadata update API, here is the
link:
I18ad1ac22c6aee298ccc75c978b85241ae1ac616

Fixes: bug #1196893

Change-Id: I788782a9f21ec2672551f75123753175bb268586
Merge "Fix multi availability zone issue part 1"