Add mocks to fix unit tests
Add mocks to fix unit tests

Change: https://review.opendev.org/687305 broke
the unit tests. We need to mock the method:
  get_undercloud_registry()

Change-Id: I2488a8bdda1eab8f46d5dfda027633174f52e342
Closes-Bug: #1848275

openstack/python-tripleoclient