vtep: Update vtep schema with tunnel table

Added a Tunnel table to the VTEP schema that allows
per-tunnel BFD configuration and status to be specified.
Removed the BFD configuration/status from the
Physical_Locator table.

Signed-off-by: Ashwin Swaminathan <ashwinds@arista.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Bruce Davie <bdavie@vmware.com>