oslo_cache.memcache_pool uses the same dogpile.cache but adds support of
pools, which may mark one memcached backend as dead, when instance
is down. os_nova uses oslo_cache.memcache_pool for some time,
so this patch adds some unification for our roles as well.
Change-Id: Ice9a806aa4c6ab60c665f35b7197555f405ac0eb