Split test case in ironic.tests.conductor.test_manager

Currently ManagerTestCase is really huge and is quite hard to
navigate in. This patch creates one more common mix-in for
setUp and splits this test case into several smaller.

Also fixes usage of mock in setUp.

Change-Id: Ib1b69723fe595dfd4d8c7f118b91a59eda7001a6
Merge "Split test case in ironic.tests.conductor.test_manager"