Add pluggability for matchmaker-redis

This commit introduces stevedore to matchmaker-redis:

https://review.openstack.org/#/c/161615/

Change-Id: I547157c02c9e0536add6949910a911540f27fb2d
diff --git a/lib/rpc_backend b/lib/rpc_backend
index 3d4ef76..8a4a090 100644
--- a/lib/rpc_backend
+++ b/lib/rpc_backend
@@ -251,8 +251,7 @@
         iniset $file $section rpc_backend "zmq"
         iniset $file $section rpc_zmq_host `hostname`
         if [ "$ZEROMQ_MATCHMAKER" == "redis" ]; then
-            iniset $file $section rpc_zmq_matchmaker \
-                oslo.messaging._drivers.matchmaker_redis.MatchMakerRedis
+            iniset $file $section rpc_zmq_matchmaker "redis"
             MATCHMAKER_REDIS_HOST=${MATCHMAKER_REDIS_HOST:-127.0.0.1}
             iniset $file matchmaker_redis host $MATCHMAKER_REDIS_HOST
         else