[rabbit] use retry parameters during notification sending

The rabbit backend now applies the [oslo_messaging_notifications]retry,
[oslo_messaging_rabbit]rabbit_retry_interval, rabbit_retry_backoff and
rabbit_interval_max configuration parameters when tries to establish the
connection to the message bus during notification sending.

This patch also clarifies the differences between the behavior
of the kafka and the rabbit drivers in this regard.

Closes-Bug: #1917645
Change-Id: Id4ccafc95314c86ae918336e42cca64a6acd4d94