Implement hash fields select group

Implement hash fields select group

This is intended as a usable demonstration of how
the NTR selection method extension might may be used.

NTR selection method
Signed-off-by: Simon Horman <simon.horman@netronome.com>
[blp@nicira.com added a NEWS entry]
Signed-off-by: Ben Pfaff <blp@nicira.com>