Split and fix availability zone filter unit tests

The availability zone filter unit tests were broken, but the problem
with them was being masked due to aggregate fixtures that were created
in tests that happened to be alphabetically ordered right before them.
This patch splits out these unit tests into their own file and mocks out
the appropriate call to nova.db.aggregate_metadata_get_by_host() instead
of relying on database records.

Change-Id: Ic692d3e7c77047ae6d901b955bc504fdc5e73738
Merge "Split and fix availability zone filter unit tests"