Tolerant behavior in case of RabbitMQ connection problem

- do not requeue task if we have problem with RabbitMQ;
- prevent throttle if only one worker exist.

Notice: after this change Nailgun will not fail task if
Astute perform unexpected exit because we do not requeue
task. In this case we will again see dangling deploy
instead of incorrect error.

Special thanks to Evgeniy L for help.
Backport from 5.0

Change-Id: Ic2e771a6133eb0f95e478417f30c1398647c2597
Closes-Bug: #1316761