Change the level of mocking for the wait_for_stack_ready test
Change the level of mocking for the wait_for_stack_ready test

This change fixes the tripleoclient tests, which were broken
by an update in heatclient.

Previously it mocked something that was called by heatclient,
this is out of our control so we can't tell when the logic
changes. This patch updates the test to mock at our bounardy
which is the call to heatclient.

Change-Id: I0b959560ee7df194ec0f4a2b8a1605666fdec551

openstack/python-tripleoclient