Add a driver method specifically for sending notifications

Notifications are an unusual case in that we need users to manually opt
in to new incompatible message formats by editing configuration because
there may be external consumers expecting the old format.

Add a send_notification() method to the driver interface and add a
format version paramater to the method, to make it clear that this
version selection is specifically for notifications.

In the case of the rabbit/qpid drivers, the 2.0 format is where we added
the message envelope.

Change-Id: Ib4925c308b1252503749962aa16f043281f2b429