FEATURE: gcli commands "firefoxos port get/set/reset" to manage Simulator preferred Remote Debugger port

- firefoxos port set NUMBER: set a persistent preferred simulator port
- firefoxos port get:
  - if simulator is running: print current listening port
  - if simulator is not running: print preferred port if any
- firefoxos port reset: reset the persistent preferred simulator port

when simulator is started:
- if a preferred port is set, try to use the preferred port
  - if preferred port can't be used find a new port and notify user
  - if preferred port can be used, simulator will listen on the
    preferred port
- if a preferred port is not set, silently find a free port