Cluster two separate apps together

[jesseshieh]

I tested this by

1. creating two gigalixir apps: bar and easygoing-impolite-bee
2. set the MY_COOKIE config on both apps to be the same e.g.

   gigalixir set_config bar MY_COOKIE "secretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecret"
   gigalixir set_config easygoing-impolite-bee MY_COOKIE "secretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecretsecret"
   

3. hit https://bar.gigalixirapp.com/nodes and saw 2 nodes listed

   {"data":["bar@10.244.7.79","bar@10.244.6.81"]}
   

4. hit easygoing-impolite-bee.gigalixirapp.com/connect?url=https://bar.gigalixirapp.com/nodes to connect easygoing-impolite-bee with bar.
5. hit https://bar.gigalixirapp.com/nodes and saw 3 nodes listed one being an easygoing-impolite-bee node

   {"data":["bar@10.244.7.79","easygoing-impolite-bee@10.244.4.114","bar@10.244.6.81"]}