cluster: add local_node settings

When overriding local node name (using NODENAME environment variable),
clustering does not work as tinit_node is tested again hostname or
fqdn. By adding the optional local_node to specify which node is the
current node we can fix this issue.

Use case: a server with a public and a private interface, rabbitmq
cluster nodes communicates through the private interface while hostname
and fqdn resolves to the public interface (and that can't be changed
because of other dependencies on the server). The only solution is to
use dedictated name for the private interface.

Author: fatpat