ovn-controller-vtep: Support BUM traffic for the VTEP Schema.

ovn-controller-vtep: Support BUM traffic for the VTEP Schema.

This patch implements BUM support in the VTEP schema.  This relates to
BUM traffic flowing from a gateway towards HVs.  This code would be
relevant to HW gateways and the ovs-vtep simulator. In order to do this,
the mcast macs remote table in the VTEP schema is populated based on the
OVN SB port binding.  For each logical switch, the SB port bindings are
queried to find all the physical locators to send BUM traffic to and the
VTEP DB is updated.

Some test packets were enabled in the HW gateway test case to exercise
the new code.

Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>