doc: Document proposed OVN Gateway HA design.

doc: Document proposed OVN Gateway HA design.

High availability for gateways in network virtualization deployments
is fairly difficult to get right.  There are a ton of options, most of
which are too complicated or perform badly.  To help solve this
problem, this patch proposes an HA design based on some of the lessons
learned building similar systems.  The hope is that it can be used as
a starting point for design discussions and an eventual
implementation.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>