Add support for OpenFlow 1.4+ "importance" values.

Add support for OpenFlow 1.4+ "importance" values.

This patch enables a user to set importance for a new rule via add-flow
OF1.4+ in the OVS and display the same via dump-flows command OF1.4+.
The changes are made in accordance with OpenFlow 1.4 specs to implement
eviction on the basis of "importance".  This patch also enhances the
diff-flows & replace-flows CLI for addition of importance parameter in
a rule.

This doesn't actually implement eviction on the basis of importance, which
will happen in a later patch.

Signed-off-by: Rishi Bamba <rishi.bamba@tcs.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>