Restore keystone cache.memcache_pool backend

Restore lost change https://review.openstack.org/#/c/118382/
Plus refactor to setup valid keystone.cache.memcache_pool params.

Closes-bug: #1340657
Change-Id: Id30ad74afe8cef17a6c42f6dfc5c9c581348bc1f
Merge "Restore keystone cache.memcache_pool backend"