Fix Murano action _get_fake_class
Fix Murano action _get_fake_class

This updates the _get_fake_client for the Murano action so
that openstack action generation works correctly for this
set of actions.

Change-Id: I85e27aea0c299f32d54597d3e922e2ee0cfe19b2
Closes-Bug: #1599854

openstack/mistral,openstack/mistral,StackStorm/mistral,StackStorm/mistral