Split agg image props isolation filter unit tests

Breaks out the unit tests for aggregate image properties isolation
filters from test_host_filters.py. In the process, we remove use of the
database and mock out the calls to db.aggregate_metadata_get_by_host(),
which results in the reduction of unit test times.

Change-Id: Ie6742fcb26364684e4a998168d14a7f29e238edb
Merge "Split agg image props isolation filter unit tests"