21->26 flow missing for NAT use case

21->26 flow missing for NAT use case

Description
-----------
21->26 flow is missing with the below use case

Use case :

1. create network
2. create subnet
3. create vm
4. create router
5. add subnet to router
6. create external network and subnet
7. create external bgpvpn
8. associate external bgpvpn to external network
9. openstack router set router1 --external-gateway myextnet10
       --disable-snat
10. openstack router set router1 --external-gateway myextnet10
       --enable-snat

Notice that the 21->26 flow is missing

Changes are done to install the 21->26 flows when we disable and
enable SNAT

Issue : NETVIRT-1294

Change-Id: I39a2b4d20aec4694aa269c334dc5635e7d7acd7e
Signed-off-by: shaik <shaik.b@altencalsoftlabs.com>