ovn-controller: Move local_datapaths calculation.

ovn-controller: Move local_datapaths calculation.

Before this patch, physical.c build up the set of local datapaths for
its own use.  I'd like to use it in another module in a later patch, so
pull it out of physical.  It's now populated by the bindings module,
since that seems like a more appropriate place to do it, and it's also
done much earlier in the main loop, making it easier to re-use.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Han Zhou <zhouhan@gmail.com>
Acked-by: Ben Pfaff <blp@ovn.org>