Move LockFixture into a fixtures module

In Nova and other projects, fixtures are in test-requirements.txt and
NOT in requirements.txt so updating nova to use latest lockutils.py
is not possible. So let us move LockFixture out into it a separate
fixtures.py module where in theory we can add other fixtures as
well. See https://review.openstack.org/#/c/52153/ for an attempt to
try latest lockutils.py in Nova.

Change-Id: I12942d6390d06bb889d4145e708692fa886d99b5
Merge "Move LockFixture into a fixtures module"