ofp-actions: Pretend that OpenFlow 1.0 has instructions.

This allows callers to be more uniform, because they don't have to pick
out whether they should parse actions or instructions based on the OpenFlow
version in use.  It also allows the Write-Metadata instruction emulation
in OpenFlow 1.0 to be treated just as in OpenFlow 1.1 in the sense that
it is allowed in contexts where instructions are allowed in OpenFlow 1.1
and not elsewhere.  (The changes in the tests reflect this more accurate
treatment.)

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>