ofp-actions: Distinguish OF1.1/1.2 push_mpls from OF1.3+.

ofp-actions: Distinguish OF1.1/1.2 push_mpls from OF1.3+.

In OpenFlow 1.1 and 1.2, the push_mpls action pushes the MPLS label after
any existing VLAN tag.  In OpenFlow 1.3, it pushes the label before any
existing VLAN tag.  Until now, the action parser didn't distinguish these
cases.  This commit adds support.  Nothing yet actually changes the
behavior of push_mpls.

enum ofpact_mpls_position contributed by Ben Pfaff.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>