Role change command support 

The changes include parsing of role request sent at the the time of controller initialization as a command line argument, addition of role change API calls that tell controller role to the OpenFlow devices, and addition of error event handler printing error type, code and message when error is thrown.

Currently a new command line argument has been added to ryu code and is not a part of rfproxy. An issue can be opened regarding porting it to rfproxy.