ovn: Add logical_datapath concept to schema.

ovn: Add logical_datapath concept to schema.

It's possible that we could implement all of the OVN concepts as a single
collection of flow tables, but it makes more sense to me if we instead
represent each logical switch and logical router separately.  This also
fits the model described in T. Koponen et al., "Network Virtualization in
Multi-tenant Data Centers," NSDI '14, the model that OVN aspires to
imitate.

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