Separate disk filters out from test_host_filters

Simply moves the disk filters out of the test_host_filters.py file and
into their own test file, removing some copy/paste cruft while we're at
it.

Change-Id: I5c83ac1a9fe6c7b6f4019a29d1a5a5847edf1723
Merge "Separate disk filters out from test_host_filters"