Support decoding of NTR selection method

Support decoding of NTR selection method

This is in preparation for supporting group mod and desc reply
messages with an NTR selection method group experimenter property.

Currently decoding always fails as it only allows properties for known
selection methods and no selection methods are known yet. A subsequent
patch will propose a hash selection method.

NTR selection method
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>