Default RABBIT_HOST to be the SERVICE_HOST.

Using localhost for RABBIT_HOST can cause problems - particularly
on systems where IPv6 is partially broken and localhost resolves
to an IPv6 address.

The service should logically be running on SERVICE_HOST unless
otherwise overridden.

Change-Id: I6efb71b8b95174770066f38403d5d428ec88c3b6
diff --git a/stack.sh b/stack.sh
index 91f188f..b4d30ee 100755
--- a/stack.sh
+++ b/stack.sh
@@ -425,7 +425,7 @@
 
 # Rabbit connection info
 if is_service_enabled rabbit; then
-    RABBIT_HOST=${RABBIT_HOST:-localhost}
+    RABBIT_HOST=${RABBIT_HOST:-$SERVICE_HOST}
     read_password RABBIT_PASSWORD "ENTER A PASSWORD TO USE FOR RABBIT."
 fi