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).
Merge pull request #130 from FlowForwarding/issue124

Fix validating ofp_action_output.max_len; introduce related test scenarios