Fixing availability-zone not take effect error

when add/remove a host to one aggregate or update aggregate
metadata incluing availability_zone, "OS-EXT-AZ:availability_zone"
property of some instances in the host can not show correctly.

The cause is that when getting availability_zone of one instance,
it will try to get the value from cache first, but unfortunately
the cache does not update or reset in time, and it will keep one
hour if we do not change it.

This patch will add update or reset after adding/removing a host
to one aggregate or updating aggregate metadata including
availability_zone.

Change-Id: I5dd07f876471b5faf8fb1016e25a861124b7cb6f
Closes-bug: #1240374
Merge "Fixing availability-zone not take effect error"