Allow test_lockutils to run in isolation

While trying to port oslo to python 3, we discovered that test_lockutils
wasn't working when not run with the global test suite. The fix can be
tested running:

$ python -m openstack.common.lockutils python -m testtools.run \
  tests.unit.test_lockutils

Change-Id: I51461f63cdd8e7eb6f8b6fdfa838748a582ecdad
Merge "Allow test_lockutils to run in isolation"