Get test_openAuth_can_refuse_None_uri to cleanup after itself

test_openAuth_can_refuse_None_uri sets the global
libvirt.allow_default_uri_connection to False but it leaves
it in this state which could cause test_tpool_execute_calls_libvirt
to fail if it ran immediately afterwards. This patch adds a cleanup
so _reset is called at the end of the test and not just if
another test in the same class is run after it.

Change-Id: I9762fcb81b2318beeb056c67bd9172056e1449ab
Closes-Bug: 1258553
Merge "Get test_openAuth_can_refuse_None_uri to cleanup after itself"