According to the latest keystone configuration help,
dogpile.cache.memcached is more recommended option in TripleO
deployment than oslo_cache.memcache_pool, because it uses httpd+wsgi
to run apis and has less than 100 threaded servers.
This patch replaces backend used in nova caching, and also introduces
the new parameter tripleo::profile::base::nova::cache_backend so that
operators can use another backend if they want.
(cherry picked from commit