Separate filter unit tests for agg extra specs

This patch separates the unit tests for the aggregate instance extra
specs filters out from test_host_filters.py. In the process, it converts
the unit tests to NoDBTestCase and mocks out any database calls.

Change-Id: I7b9f186052825667b585674491d0ddf6b566f8de
Merge "Separate filter unit tests for agg extra specs"