Fix errors with issubclass() in Python 3.7
Fix errors with issubclass() in Python 3.7

The cfn Ref instrinsic function is implemented as a Python function
rather than a class, so calling issubclass() on it fails in Python 3.7.

Change-Id: I3f826cd8f024b591d7c3b41ad1e3f960558d3f9b

noironetworks/heat,noironetworks/heat,openstack/heat,openstack/heat