Add Glusterfs volume for keystone credential-keys.

Change-Id: Ife24619123ae962aac9d58dca4a5cdee125e26af
diff --git a/keystone/server/cluster.yml b/keystone/server/cluster.yml
index f077faf..a8f7069 100644
--- a/keystone/server/cluster.yml
+++ b/keystone/server/cluster.yml
@@ -31,6 +31,8 @@
         expiration: 3600
         max_active_keys: 3
         location: /var/lib/keystone/fernet-keys
+      credential:
+        location: /var/lib/keystone/credential-keys
       message_queue:
         engine: rabbitmq
         members: