Update availability_zone on time if it was changed

The problem is that when the availability_zones for one host
aggregation was updated, the VM attribute of availability_zones
which belong to the host in the availability_zones was
not updated on time.

If updated values include availability_zones, then the cache
which stored availability_zones and host need to be reset.

Fix bug 1203250

Change-Id: Ib3f08fbe3c74d5aa101da78faed43b0bc03cb988
Merge "Update availability_zone on time if it was changed"