Revert config mocking to cfg.CONF.set_override

After moving to a different mocking strategy, it was pointed out that
the issue was actually a missing config group import, so partially
revert the change made under 9544b80 to simplify the test and add
the missing config group import.

Related-Bug: #1273126
Change-Id: I3a8b65ab16845be060d6df4d94ae08f2d7d8255a
Merge "Revert config mocking to cfg.CONF.set_override"