Fix multi-host deployments using RabbitMQ

Currently the rabbit password and rabbit_host are no longer
written into nova.conf. This is due to this change:
https://review.openstack.org/#/c/6501/

My solution is to write the values if:
- they are set
- and you are not using the alternative queue system

Change-Id: I8de7b57125d1fdf50044fc2b3ae4683ac15d5a61
diff --git a/stack.sh b/stack.sh
index 45e285c..c86017e 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1672,11 +1672,11 @@
 add_nova_opt "api_paste_config=$NOVA_CONF_DIR/api-paste.ini"
 add_nova_opt "image_service=nova.image.glance.GlanceImageService"
 add_nova_opt "ec2_dmz_host=$EC2_DMZ_HOST"
-if is_service_enabled rabbit ; then
+if is_service_enabled qpid ; then
+    add_nova_opt "rpc_backend=nova.rpc.impl_qpid"
+elif [ -n "$RABBIT_HOST" ] &&  [ -n "$RABBIT_PASSWORD" ]; then
     add_nova_opt "rabbit_host=$RABBIT_HOST"
     add_nova_opt "rabbit_password=$RABBIT_PASSWORD"
-elif is_service_enabled qpid ; then
-    add_nova_opt "rpc_backend=nova.rpc.impl_qpid"
 fi
 add_nova_opt "glance_api_servers=$GLANCE_HOSTPORT"
 add_nova_opt "force_dhcp_release=True"