ofp-util: Break out encoding of OpenFlow 1.1 group mod messages.

This refactoring is in preparation for supporting encoding
of (draft) OpenFlow 1.5 group mod messages.

ONF-JIRA: EXT-350
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>