RabbitMQ: advance thru the list of brokers on reconnect

In RabbitMQ implementation, when using multiple rabbit_hosts, we don't want to
immediately retry failed connection for the same failed broker. This was not
the case in existing implementation though, where we've always attempted to
reconnect starting from the first broker in the list of candidates. So if the
first broker failed, we initiated reconnect to the same failed broker.

This change makes reconnect() implementation to select the next broker in the
list. This also means that non-failure reconnect attempts will also switch the
current broker. All in all, users should not rely on any particular order to
use brokers from the list, so this should not constitute an issue.

This is a backport of 71c6866471d628b207d7c5b84bfd37cc9fed0898 from
oslo.messaging.

Change-Id: I67923cb024bbd143edc8edccf35b9b400df31eb3
Partial-Bug: 1261631
Merge "RabbitMQ: advance thru the list of brokers on reconnect"