Refactor unit tests for scheduler weights

Remove external depenencies from scheduler weight tests in an attempt
to make the tests cleaner and to only test the weight code itself.

FakeHostState() is used to avoid having to go through host manager and
then mock it's db calls. Additionally, weigher classes are referenced
directly instead of having the weight handler load them first.

Related to blueprint scheduler-optimization

Change-Id: Ifd3577aec6c5fc82e4864623e0545f380fb75b6e
Merge "Refactor unit tests for scheduler weights"