Fix AvailabilityZone check for hosts in multiple aggregates

Check for multiple AZ was distinct when adding an host to an aggregate
and when updating an aggregate (or its metadata), which was leading to
possible corner-cases where logics were different.

This patch proposes a single way for checking for both API methods and
also adds an extra control on the possibility to have an aggregate with
its metadata AZ set to the configuration default value (eg. 'nova') and
where it was possible to subsequently have one host in two disctinct AZs
Fixed that scenerio as it is a gap, Nova as of now preventing to have
one host in two distinct AZs.

Change-Id: Ida6aa507f34f36a436adaafe1f7609963c59a9bc
Partial-Bug: #1277230
Merge "Fix AvailabilityZone check for hosts in multiple aggregates"