ofp-actions: Implement writing to metadata field

ofp-actions: Implement writing to metadata field

In OpenFlow 1.1, we add support for OFPIT_WRITE_METADATA. This allows us to
write to the metadata field. Internally it is represented using ofpact_metadata.

We introduce NXAST_WRITE_METADATA to handle writing to the metadata field in
OpenFlow 1.0+. This structure reflects OFPIT_WRITE_METADATA.

When writing out the structure to OpenFlow 1.1, it uses the OFPIT_WRITE_METADATA
instruction only, and not the new NXAST action (which would be redundant).

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>