cluster_formation.peer_discovery_backend aliases

[michaelklishin]

This plugin's name is rabbitmq_peer_discovery_etcd but the module name that must be configured for discovery is

cluster_formation.peer_discovery_backend = rabbit_peer_discovery_etcd

which confuses some users and leads to hours spent troubleshooting and a lot of frustration.

We should introduce an alias module and consider an alias shortcut supported in rabbitmq.conf so that these config values

cluster_formation.peer_discovery_backend = rabbitmq_peer_discovery_etcd

cluster_formation.peer_discovery_backend = etcd

would be accepted.

[michaelklishin]

FTR, short names have been supported since 3.7.0: rabbitmq/rabbitmq-server@5f9752c, rabbitmq/rabbitmq-server@245e3c7