Add specifying a controller in the config generator

To make config_gen script more useful, especially for Mininet,
generation of controllers' specifications was added.
(Motivated by #199).
Improve config generation script help

Example usage is added to the help information for config_gen scrip.
(Motivated by #199).
Print error message for missing arguments

Previously, this would just give a {badmatch,false} error.
Merge pull request #200 from FlowForwarding/issue199

Improve config_gen script