Allow different login methods to be used with kombu connections

Currently it is not possible to modify the login method used with
kombu connections via configuration files, and requires modifying
the source code to actually add a login method in the first place.
This adds a configuration option (and default) which are passed
along to the kombu connection.

RabbitMQ allows three login method strings by default, but server
side plugins allow arbitrarily named strings to be used as login
methods and as such it is not possible to provide a fixed list of
options to the user.

DocImpact

Change-Id: I5c62a8dd4125d42bb429d3985061e37a9d519fff
Closes-Bug: #1269890
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.

Change-Id: I76965ea06c1fb6fd0cd384a5b3edc14703470b8f
Partial-Bug: 1261631
Qpid: advance thru the list of brokers on reconnect

In Qpid implementation, when using multiple qpid_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.

Change-Id: I257c2ad6d7ebead356c0239134340975da6dbc07
Partial-Bug: 1261631
Merge "Allow different login methods to be used with kombu connections"
Merge "Qpid: advance thru the list of brokers on reconnect"
Merge "RabbitMQ: advance thru the list of brokers on reconnect"