ofp-util: Provide helper to encode OpenFlow 1.1 group buckets.

The group bucket encoding code appears in two places
in almost identical form.  Consolidate this into
a helper function.

The helper name includes ofp11 as later patches
will add an ofp15 version to support (draft) OpenFlow 1.5 buckets.

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