Use MEMCACHE_SERVERS in configure_keystone_authtoken_middleware
Use MEMCACHE_SERVERS in configure_keystone_authtoken_middleware

The function was using a hard coded value of localhost:11211 when
we have an option MEMCACHE_SERVERS that can be defined and used
inside DevStack.

Change-Id: I4947928fe406a9844d5bdaa3c826d273952fa097

openstack-dev/devstack,nawawi/openstack,openstack-dev/devstack,nawawi/openstack