Check results of joining to the cluster
Check results of joining to the cluster

Joining cluster may fail, so we need to handle failure according to
`autocluster_failure` setting.

Before https://github.com/rabbitmq/rabbitmq-server/pull/868 even `{ok,
already_member}` should be treated as an error, but I'm not so sure
about this.

aweber/rabbitmq-autocluster,alanprot/rabbitmq-autocluster,rabbitmq/rabbitmq-autocluster