Remove support for the legacy 'cluster_nodes' values

Before this change, a list of nodes without the node type was accepted.
In this case, the node type was guessed and a warning suggesting how to
update the configuration was logged.

Now, the node type is mandatory and the RabbitMQ server refuses to start
if the node type is unspecified.
Detect other invalid 'cluster_nodes' values

In all cases, abort startup with an explanatory message.
Merge bug26257