ovs-vswitchd: Log datapath ID in a more user-friendly way.

ovs-vswitchd: Log datapath ID in a more user-friendly way.

The layering between ofproto and ovs-vswitchd caused the datapath ID to be
logged in a needlessly confusing way.  First, ofproto would log its
default datapath ID:

     using datapath ID 0000505400000004

then the bridge code would immediately determine the datapath ID that it
wanted and call ofproto_set_datapath_id(), which would log the change

     datapath ID changed to 0000111122223333

This commit stops logging the default datapath ID, which is never actually
visible in OpenFlow.  This should make the log files easier to understand.

Bug #12164.
Reported-by: Jacob Cherkas <jcherkas@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>