Switch keystone to dogpile.cache.memcached backend
Switch keystone to dogpile.cache.memcached backend

memcache_pool is there to keep a limited number of thread-associated
connections open rather than a connection for every thread. If you
don't have a huge number of threads it doesn't offer anything.
Keystone is an example of a service where memcache_pool doesn't
improve things -- eventlet isn't supported anymore and more threads
is not useful due to GIL.

As such, keystone cache backend is changed to dogpile.cache.memcached.

See https://review.openstack.org/357407 for the oslo.cache help text
change.

Change-Id: I4452a8c4968073cdea4c0f384453a5a28519fa08

noironetworks/devstack,openstack-dev/devstack,nawawi/openstack,openstack-dev/devstack,bswartz/devstack,noironetworks/devstack,richliu/devstack,group-policy/devstack,dtroyer/devstack,dtroyer/devstack,richliu/devstack,nawawi/openstack,bswartz/devstack,group-policy/devstack