vtep: Support per-tunnel tunnel key in schema.

vtep: Support per-tunnel tunnel key in schema.

Currently the scenario of single logical bridge that use multiple locators
with different tunnel key on each of the locators is not supported.
In order to support much more flexibility in the tunnel settings, we need
to add tunnel key column to the Physical_Locator table.

This patch is needed to support the usage of neutron L2GW as an inter-cloud
gateway. The tunnel key that is set in the logical bridge will be used
'internally' to connect the L2GW to the compute hosts and the key that
is set in the physical locator will be used to connect the L2GW to a
remote L2GW to form a tunnel between 2 clouds. In this case, the
'external' connection will use single 'dst_ip' on all the locators with
different tunnel key for each L2 domain.

The Neutron spec is available here:
	https://review.openstack.org/#/c/270786/
The new code depend on the ability to use different keys in multiple
tunnels in the same logical switch.

Signed-off-by: Ofer Ben-Yacov <ofer.benyacov@gmail.com>
[blp@ovn.org added and clarified documentation]
Signed-off-by: Ben Pfaff <blp@ovn.org>