Improve performance for checking hosts AZs

When updating aggregate metadata or when adding a host into aggregate,
a check is done to make sure that hosts can't be in multiple AZs.

In order to do the check, the routine was looping over each host to look
in the DB which AZs the host is part of.
As it's quite hungry in terms of DB accesses, here the proposal is to
extend the helper method for getting all AZs by also having their hosts,
and so only do memory iterations.

Partial-Bug: #1277230

Change-Id: I7c5bc980acb17f0a0e1f8c1403dac66c305866fb
Merge "Improve performance for checking hosts AZs"