ofp-actions: Support "resubmit" actions in action sets.

Fixing issue where "resubmit" action in a group action set was not
considered sufficient to retain the full action set. This patch allows
a group action set (considered terminal with OF1.4 and earlier spec)
to have the "output" action come from a different table.

Signed-off-by: Srini Seetharaman <srini.seetharaman@gmail.com>
[blp@nicira.com added documentation]
Signed-off-by: Ben Pfaff <blp@nicira.com>