Allow opening cluster port on reload

[wallyqs]

Feature Request

Add support to enable cluster mode when starting from standalone single server mode.

Basically going from this:

port = 4222

To this:

port = 4222

cluster {
  port = 6222
}

Use Case:

Start from single server then enabling cluster port to act as a seed.

Proposed Change:

Who Benefits From The Change(s)?

• Those looking at dynamically modifying the clustering of the server

Alternative Approaches

• Start the server with the cluster port enabled but without routes
• Restarting the server

[derekcollison]

This not work?