Fix broken unit test
Fix broken unit test

This change mocks sqlalchemy.update during unit tests properly to avoid
the following error.

sqlalchemy.exc.ArgumentError: subject table for an INSERT, UPDATE or
DELETE expected, got <MagicMock name='models.Instance' id='...'>

Closes-Bug: #1938676
Change-Id: I5268132018e0a283bd35b5599cf8ca41968dde93

openstack/murano,openstack/murano