ofp-parse: Parse (draft) OpenFlow 1.5 bucket ids.

This is part of support for (draft) OpenFlow 1.5 flow mod messages.

This adds support for specifying the bucket_id of buckets of groups in
ovs-ofctl and documents that accordingly.

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