nicira-ext: New Nicira vendor action NXAST_NOTE.

nicira-ext: New Nicira vendor action NXAST_NOTE.

Our controller group at Nicira has requested a way to annotate flows with
extra information beyond the flow cookie.  The new NXAST_NOTE action
provides such a way.

This new action is somewhat controversial.  Some have suggested that it
should be added another way (either as part of the Nicira Extended Match
or as a new component of the flow_mod and related messages).  Others think
that it has no place in the OpenFlow protocol at all and that an equivalent
should be implemented using the already available features of OVSDB.  So
it is possible that this extension will be deleted and the feature will
be reimplemented some other way (or not at all).

CC: Teemu Koponen <koponen@nicira.com>
CC: Jeremy Stribling <strib@nicira.com>