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
diff --git a/lib/keystone b/lib/keystone
index 6198e43..6d8a43c 100644
--- a/lib/keystone
+++ b/lib/keystone
@@ -245,7 +245,7 @@
 
     # Enable caching
     iniset $KEYSTONE_CONF cache enabled "True"
-    iniset $KEYSTONE_CONF cache backend "oslo_cache.memcache_pool"
+    iniset $KEYSTONE_CONF cache backend "dogpile.cache.memcached"
     iniset $KEYSTONE_CONF cache memcache_servers localhost:11211
 
     # Do not cache the catalog backend due to https://bugs.launchpad.net/keystone/+bug/1537617