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
diff --git a/lib/keystone b/lib/keystone
index 9ceb829..366e6c7 100644
--- a/lib/keystone
+++ b/lib/keystone
@@ -421,7 +421,7 @@
     iniset $conf_file $section project_domain_name "$SERVICE_DOMAIN_NAME"
 
     iniset $conf_file $section cafile $SSL_BUNDLE_FILE
-    iniset $conf_file $section memcached_servers localhost:11211
+    iniset $conf_file $section memcached_servers $MEMCACHE_SERVERS
 }
 
 # configure_auth_token_middleware conf_file admin_user IGNORED [section]