Separate and refactor affinity filter tests

The unit tests for the scheduler affinity tests were actually *not*
NoDBTestCase tests. In fact, they were relying on the database to create
instance objects instead of just mocking out the call to
nova.compute.api.get_all(). This patch corrects that and separates out
the affinity filter unit tests out of test_host_filters.py.

Change-Id: Ib7268a49e115fbeff7f18c569cecd82b3d3e8cb3
Merge "Separate and refactor affinity filter tests"