ovs-vtep: Add support for bfd tunnels.

The VTEP emulator creates one OVS bridge for every logical switch and then
programs flow in it based on learned local macs and controller programmed
remote macs.

Multiple logical switches can have multiple OVS tunnels to the
same remote machine (with different tunnel ids). But VTEP schema expects
a single BFD session between two physical locators. Therefore
create a separate bridge ('bfd_bridge') and create a single OVS tunnel
between two physical locators (using reference counter).

The creation of BFD tunnels by the VTEP emulator is mostly for reporting
purposes. That is, it can be used by the controller to figure out that
a remote port is down. The emulator itself does not base any of its
forwarding decisions based on the state of a bfd tunnel.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ariel Tubaltsev <atubaltsev@vmware.com>