Use fixtures library for nova test fixtures.
Use fixtures library for nova test fixtures.

Moving the test-case setup and teardown code into managed fixtures
so that we can better interact with them when we start running things
in parallel.

Part of blueprint grizzly-testtools

Change-Id: I406be0a88b14c0dba2d22b4957e26a53442bbae3

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects