title: cluster_formation.classic_config.nodes does not work of rabbitmq
body: <p>I have 2 rabbitmq nodes.
  Their node names are: rabbit@testhost1 and rabbit@testhost2
  I'd like them can auto cluster.</p>
  
  <p>On testhost1</p>
  
  <pre><code># cat /etc/rabbitmq/rabbitmq.conf
  cluster_formation.peer_discovery_backend = rabbit_peer_discovery_classic_config
  cluster_formation.classic_config.nodes.1 = rabbit@testhost1
  cluster_formation.classic_config.nodes.2 = rabbit@testhost2
  </code></pre>
  
  <p>On testhost2</p>
  
  <pre><code># cat /etc/rabbitmq/rabbitmq.conf
  cluster_formation.peer_discovery_backend = rabbit_peer_discovery_classic_config
  cluster_formation.classic_config.nodes.1 = rabbit@testhost1
  cluster_formation.classic_config.nodes.2 = rabbit@testhost2
  </code></pre>
  
  <p>I start rabbit@testhost1 first and then rabbit@testhost2.</p>
  
  <p>The second node didn't join to the cluster of first node.</p>
  
  <p>While node rabbit@testhost1 can join rabbit@testhost2 with rabbitmqctl command: rabbitmqctl join_cluster rabbit@testhost2.
  So the network between should not have problem.</p>
  
  <p>Could you give me some idea about why can't combine cluster? Is the configuration nor correct?</p>
  
  <p>I have opened the debug log and the info related to rabbit_peer_discovery_classic_config is very little:</p>
  
  <pre><code>2019-01-28 16:56:47.913 [info] &lt;0.250.0&gt; Peer discovery backend rabbit_peer_discovery_classic_config does not support registration, skipping registration.
  </code></pre>
  
  <p>The rabbitmq version is 3.7.8</p>