Do not reference ansible_hostname if it is undefined
Do not reference ansible_hostname if it is undefined

Change-Id: I59c3de74c41b97ed1803d82c69db52199f27393e
Closes-Bug: #1797117

openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates