Disable ipv6_subnet_attributes feature in case of contrail env

"ipv6_ra_mode" and "ipv6_address_mode" fields are not supported
by OpenContrail (at least in 4.0). Related bug:
https://bugs.launchpad.net/juniperopenstack/+bug/1604568

Change-Id: I95a4e2bd46b6d357c2474187bed9d23488be42ba
Related-PROD: PROD-22168
1 file changed