Select AMQP message broker at random

The rule of choosing AMQP message broker is that chose first
available one in order now. The order depends on what we set in
configuration file. That means all the connections will flock
to same message broker and that may lead out performance issue.
This patch randomizes the order of choosing message broker for
each connection to leverage broker cluster.

Change-Id: Ib5098e574d4ef81428065885e2295d0f87aba715
Partial-Bug: #1261631