Delete the default guest user from rabbitmq

Leaving the default user enabled is a security issue, as it can be used
without credentials. It also may mask issues like seen in [1].

[1] https://bugs.launchpad.net/bugs/1651576

Change-Id: I75b4e5696c0f8017b869127a10f3c14e2f8bd121
diff --git a/lib/rpc_backend b/lib/rpc_backend
index 3177e88..5479db3 100644
--- a/lib/rpc_backend
+++ b/lib/rpc_backend
@@ -97,6 +97,8 @@
 
             break
         done
+        # NOTE(frickler): Remove the default guest user
+        sudo rabbitmqctl delete_user guest || true
     fi
 }