openvswitch: Interface with NAT.

Extend OVS conntrack interface to cover NAT.  New nested
OVS_CT_ATTR_NAT attribute may be used to include NAT with a CT action.
A bare OVS_CT_ATTR_NAT only mangles existing and expected connections.
If OVS_NAT_ATTR_SRC or OVS_NAT_ATTR_DST is included within the nested
attributes, new (non-committed/non-confirmed) connections are mangled
according to the rest of the nested attributes.

The corresponding OVS userspace patch series includes test cases (in
tests/system-traffic.at) that also serve as example uses.

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: Thomas Graf <tgraf@suug.ch>
Acked-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>