Slow down Kombu reconnect attempts

Back ported from oslo.messaging in Icehouse.
For a rationale for this patch, see the discussion surrounding Bug
When reconnecting to a RabbitMQ cluster with mirrored queues in
use, the attempt to release the connection can hang "indefinitely"
somewhere deep down in Kombu.  Blocking the thread for a bit
prior to release seems to kludge around the problem where it is
otherwise reproduceable.

Implements: rally-user-story US6888
Upstream-Review: https://review.openstack.org/#/c/76686/
Not-in-upstream: true