ofp-util: Encoding and decoding of (draft) OpenFlow 1.5 group messages.

This provides the bulk of the ofproto side of support for
OpenFlow 1.5 group messages. It provides for encoding and decoding
of updated group mod and group desc reply messages. This includes
a new bucket format and their properties.

Open Flow 1.5 Groups also have properties but as no non-experimenter
properties are defined this patch does not provide parsing or encoding
of group properties.

ONF-JIRA: EXT-350
Signed-off-by: Simon Horman <simon.horman@netronome.com>
[blp@nicira.com fixed minor bugs and style issues]
Signed-off-by: Ben Pfaff <blp@nicira.com>