Fix hosts extension and enable its tests

This patch corrects the import in hosts.py, fix the DB API
service_get_by_host_and_topic raises ServiceNotFound exception with
right parameter and move tests to the right place so that they are
enabled.

Fix LP bug: #1104231

Change-Id: I07c486f456196a3812eb5274b950ab9560a51cf3
Merge "Fix hosts extension and enable its tests"