Move RABBIT_USERID to lib/rpc_backend

This moves setting of RABBIT_USERID from stack.sh to lib/rpc_backend
so it may be used in grenade runs, which don't have the defaulted
value from stack.sh. The RABBIT_USERID is needed in order to call
get_transport_url in lib/rpc_backend.

Change-Id: I6f211e9102f79418f9f94a15784f91c4150ab8a7
diff --git a/lib/rpc_backend b/lib/rpc_backend
index 0ee46dc..97b1aa4 100644
--- a/lib/rpc_backend
+++ b/lib/rpc_backend
@@ -24,6 +24,8 @@
 _XTRACE_RPC_BACKEND=$(set +o | grep xtrace)
 set +o xtrace
 
+RABBIT_USERID=${RABBIT_USERID:-stackrabbit}
+
 # Functions
 # ---------