ovn-northd: Add stages for logical routers.

Until now, ovn-northd has only set up flows for logical switches.  With the
arrival of logical routers, it needs to set up flows for them too.  The
stages within logical routers are completely different from those for
logical switches, so this prepares for that by adding logic for identifying
those stages.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>