ofproto-dpif: Avoid creating OpenFlow ports for duplicate tunnels.

ofproto-dpif: Avoid creating OpenFlow ports for duplicate tunnels.

Until now, when two tunnels had an identical configuration, both of them
were assigned OpenFlow ports, but only one of those OpenFlow ports was
functional.  With this commit, only one of the two (or more) identically
configured tunnels will be assigned an OpenFlow port number.

Reported-by: Keith Holleman <hollemanietf@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Co-authored-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>