[odl] Set conntrack as netvirt nat mode

[odl] Set conntrack as netvirt nat mode

The conntrack-based SNAT uses the Linux netfilter framework to
do the NAPT and track the connection. The first packet in a traffic is
passed to the netfilter to be translated with the external IP. The
following packets will use the netfilter for further inbound and
outbound translation.

Change-Id: I1090b4fe041f8d9533aa4ce1964284a4a5c073ce
Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>