vtep: Add source node replication support.

vtep: Add source node replication support.

This patch updates the vtep schema, vtep-ctl commands and vtep simulator
to support source node replication in addition to service node
replication per logical switch.  The default replication mode is service
node as that was the only mode previously supported.  Source node
replication mode is optionally configurable and clearing the replication
mode implicitly sets the replication mode back to a default of service
node.

Signed-off-by: Darrell Ball <dlu998@gmail.com>
Acked-by: Bruce Davie <bdavie@vmware.com>
Acked-by: Anupam Chanda <achanda@vmware.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>