beatrix: override default tables names for external bus

Make sure that the internal and external buses don't share the same tables names
or some events won't be dispatched to the right listeners.

See discussion https://groups.google.com/forum/#!msg/killbilling-users/x3o1-EjR3V0/ZJ-PJYFM_M0J

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>