rabbit: uses kombu instead of builtin stuffs

The rabbit driver have a custom code to reconnect to a broker,
to change the broker in HA configuration, to retry a to send
a message, to handle the interval between reconnection.

But all of that exists in kombu, so just use it.

Using the kombu Connection object with the url make also the rabbit
driver more generic.

Futher patches can rename rabbit* oslo.config options to a more generic
name and add a new driver entry_point 'kombu' to allow to use this driver
with any borker supported by kombu.

Change-Id: Id6b89d5448126ca652b46fe6ce5a9b3ed5839795