Set cache_backend for token caching
Set cache_backend for token caching

'fernet' is the token provider since octata. As we've memcached
in the undercloud, we should enable caching by setting
cache_backend to 'dogpile.cache.memcached'.

Change-Id: I4b040d25b2cb83f40cbd8e8caf6890feba586b60
Closes-Bug: #1832437

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