Add support for targetPort

[c4710n]

Allow to specify remote info like this:

{
  deployment.targetHost = "11.11.11.11";
  deployment.targetUser = "deploy";
  deployment.targetPort = 8022;  # add this
}

Then:

• there's no need to use SSH_CONFIG_FILE in a simple case which just specifies a non-standard port.

I'm not an expect of Go. If you have any idea of the code, just change it. I'm ok with that. ;)

[c4710n]

I compared host.TargetPort and host.GetTargetPort() with the zero-value of int - 0.

I'm not sure whether it is a good taste of Go code. @ me if more changes are required.

[srhb]

I think it's fine even though we're relying on unmarshal defaulting to 0. If not we can always add some proper optional'ing later on.

Thank you very much! :)