Remove some duplicate unit tests

There are several locations where some unit test mixins are
incorrectly used multiple times with an otherwise same configuration,
which results in pointless re-runs of the same unit test. Similarily,
there are a few tests defined in classes inherited by multiple tests
that don't change behavior in the child tests so they are run multiple
times as well.

This patch eliminates the duplicates and moves some test cases from
their parent class into a subclass when they don't need to be run multiple
times. The result is 221 fewer test runs (~1.5% of the tests run).

Closes-Bug: #1374836
Change-Id: I292091488cd944510fa1250920d41be2a50dabaf
Merge "Remove some duplicate unit tests"
Merge branch 'master' of https://github.com/openstack/quantum