conntrack: Add support for NAT.

conntrack: Add support for NAT.

Extend OVS conntrack interface to cover NAT.  New nested NAT action
may be included with a CT action.  A bare NAT action only mangles
existing connections.  If a NAT action with src or dst range attribute
is included, new (non-committed) connections are mangled according to
the NAT attributes.

This work extends on a branch by Thomas Graf at
https://github.com/tgraf/ovs/tree/nat.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>