Sync fixture module from oslo

Changes pulled:

  [f4a4855] Consolidate the use of stubs
  [6111131] Make openstack.common.fixture.config Py3 compliant

Due to the changes to the stub use, a couple of tests (the ones in
test_shelve) needed to have some cleanups removed that depended on some
stubs still being in place and not others. The normal DB cleanup
procedure handles this correctly, so the cleanups were superfluous
anyway.

The test in test_manager was split in two to avoid it resetting its
stubs halfway through. It was testing two separate scenarios anyway, so
it's the correct thing to do regardless.

I'll submit a patch later to get rid of all the superfluous VerifyAll
and UnsetStubs calls across the entire test suite, but for now, let's
just get the gate working again, shall we?

Fixes bug #1239898

Change-Id: I3aa4dfc0261a861e5eaa570849329292d64236ed
Merge "Sync fixture module from oslo"