Fix race in unit tests, which can cause gate job to fail

The FakeLibvirtTests.test_openAuth_can_refuse_None_uri test changes the value
of the global variable allow_default_uri_connection. We need to clean up this
change up after running this test so as we don't effect other tests that
expect the value of this variable to be the default.

In particular this test was running before the
LibvirtNonblockingTestCase.test_tpool_execute_calls_libvirt which ran when
the value was changed causing this test to fail. This only occurred when we
the test_libvirt used the fake libvirt driver, which happens when the tests
run inside a virtualenv environment.

Change-Id: I68c30dffd05daae6c0d5fbc6ec005c0fab4f09be
Closes-Bug: 1258553
Merge "Fix race in unit tests, which can cause gate job to fail"