Fix validating ofp_action_output.max_len

The no_buffer value for the ofp_action_output.max_len is represented
by the atom, not the value of 0xFFFF.
(Motivated by #124).
Add scenarios testing ofp_action_output.max_len option and multiple controllers

New scenarios test if the ofp_action_outpout.max_len field is correctly
encoded/decoded and interpreted. They also introduce tests for LINC operation
under control of multiple controllers that receive filtered asynchronous
messages. For convenience new format for defining scenarios is introduced.
(Motivated by #124 and #97).
Add an empty scenario to the controller

The empty scenario is needed if we want the controller to do nothing.
(Motivated by #129).
Add a script generating a config file for the LINC

The `config_gen` script allows to provide interfaces that should be bound
to a logical switch and the output file to which the config should be written.
(Motivated by #129).