Adds tests for isolated_hosts_filter

The isolated_hosts_filter takes two configuration options. This patch
adds three tests defining the behaviour for when one or more of these
variables are missing.

This patch speeds things up a bit when the isolated_images option is
missing, making it do less dict lookups to come up with the same answer.

Matrix of isolated_hosts_filter changes:

+----------------------+--------------------+--------------+
| Config options       | Before patch       | After        |
+----------------------+--------------------+--------------+
| 0        + 0         | No filtering       | Same as B4   |
|                      | no tests           | Tested       |
+----------------------+--------------------+--------------+
| 0        + iso_image | Block iso'd images | Same as B4   |
|                      | no tests           | Tested       |
+----------------------+--------------------+--------------+
| iso_host + 0         | Block iso'd images | Same as b4   |
|                      | no tests           | Tested       |
+----------------------+--------------------+--------------+
| iso_host + iso_image | Block matched h+i  | Same as b4   |
|                      | Tested             | Tested       |
+----------------------+--------------------+--------------+

Change-Id: Ibe6725e39a771b31fdf49513335275deba33e96b
Merge "Adds tests for isolated_hosts_filter"