At the moment we don't provide any option rather then use memcached
backend. With that we also hardocde list of packages that should be
installed inside virtualenv for selected backend.
Adding bmemcached requirement to oslo_cache.memcache_pool [1] gives us
opportunity to refactor this bit of deployment and allow to be more
flexible in backend selection and requirements installation for it.
[1] https://review.opendev.org/c/openstack/oslo.cache/+/854628
Change-Id: I48e193ef29e56aa8639511c5b5dcddc70f5e1198
242ebe2b54