ofp-actions: Refactor converting ofpacts to OpenFlow 1.1 and 1.2 actions.

As we continue to add support for newer versions of OpenFlow actions, it
seems crude to add layers of wrappers around ofpact_to_openflow11().  This
commit squashes the two existing layers together.

This prepares to add the new OpenFlow 1.5 "copy-field" action.

Signed-off-by: Ben Pfaff <blp@nicira.com>
ofp-actions: Add OpenFlow 1.5 "copy-field" action.

EXT-320.
Signed-off-by: Ben Pfaff <blp@nicira.com>