refine rabbit cleanup

Beside it's good to killall -9 epmd,
if killall epmd fails

Change-Id: Ide90ef8ac3339bd70d991666ce0d3550a088670b
diff --git a/lib/rpc_backend b/lib/rpc_backend
index ff87aae..61908c4 100644
--- a/lib/rpc_backend
+++ b/lib/rpc_backend
@@ -63,7 +63,7 @@
     if is_service_enabled rabbit; then
         # Obliterate rabbitmq-server
         uninstall_package rabbitmq-server
-        sudo killall epmd
+        sudo killall epmd || sudo killall -9 epmd
         if is_ubuntu; then
             # And the Erlang runtime too
             sudo aptitude purge -y ~nerlang