Add kombu driver library to requirements.txt

The RabbitMQ driver is the default driver for oslo.messaging, so it
makes some sense to require this by default. This is consistent with
the way e.g. Nova has always required kombu.

Change-Id: Ibcab58e3f3edce794b571cfabc0a5bcf96c9b47e
Closes-Bug: #1288425
Merge "Add kombu driver library to requirements.txt"