remove unused capabilities
add helper method to get switch by it's dpid
set the controllers at the switch
refactor check_status to return the actual status of POXController
Pretty print for POXController
Fix a bug in crash/recover controller manager

ControllerManager was not keeping right track of dead/alive controllers
after crash/recovery.
Add helper method to get controller by it's ID
Fix the can_create_controller capability
Remove unnecessary print statement
inherit the right class