STUN check fixes. Re Issue #816

### Summary

 - Now STUN server is returning what should be the outer port on which
   it could listen to the OpenBazaar node (a random one)
 - The --server-port argument is ignored unless --disable-stun-check is passed.

The reason being default behavior is configured so that OpenBazaar will try to establish all its connections without user intervention.
A user that knows on what port they want OpenBazaar to run and who is manually forwarding ports should then read the help and understand the port is being ignored due to the default STUN check openbazaar attempts.

### Testing

Right now I'm behind the most restrictive NAT configuration there is so I'm still not seeing nodes, I need to test elsewhere, please test with the following launch parameters:

  Try setting a manual port, the one you have set on your router.
  `openbazaar --disable-stun-check --server-port <your_port> start`

  Try letting openbazaar do its work by looking up a valid incoming server port via STUN, and to automatically do the UPnP port mapping on such port.
   `openbazaar start`
Since you are separating the two conditions, you can refactor the previous one into just:
```python
server_port = arguments.server_port
```

I also believe you should fix the `server_ip` conditionals, right above.