Add test for _show_resource
Add test for _show_resource

Add test for situation where entity=None. In this case
_show_resource must return None.

Change-Id: I4181465bbe938d950b9245a33d5db2846c9878ca
noironetworks/heat,jasondunsmore/heat,pratikmallya/heat,dims/heat,dragorosson/heat,openstack/heat,jasondunsmore/heat,steveb/heat,cwolferh/heat-scratch,pratikmallya/heat,maestro-hybrid-cloud/heat,dims/heat,gonzolino/heat,dragorosson/heat,gonzolino/heat,maestro-hybrid-cloud/heat,openstack/heat,noironetworks/heat,steveb/heat,cwolferh/heat-scratch