Skip IPv6 Tests in the OpenContrail plugin

Similar to the way we are skipping tests in the OneConvergence plugin,
introduced by Kevin Benton in 9294de441e684a81f6e802ba0564083f1ad319d6.

Partial-Bug: #1378952

Change-Id: I1650b0708af73ce63e92c55bc842607bb69efe60
(cherry picked from commit 67962943969bc737a3f680a0defc2fc9df03c429)
Disable PUT for IPv6 subnet attributes

In Juno we are not ready for allowing the IPv6 attributes on a subnet
to be updated after the subnet is created, because:
- The implementation for supporting updates is incomplete.
- Perceived lack of usefulness, no good use cases known yet.
- Allowing updates causes more complexity in the code.
- Have not tested that radvd, dhcp, etc. behave OK after update.

Therefore, for now, we set 'allow_put' to False for the two IPv6
attributes, ipv6_ra_mode and ipv6_address_mode. This prevents the
modes from being updated via the PUT:subnets API.

Closes-bug: #1378952

Change-Id: Id6ce894d223c91421b62f82d266cfc15fa63ed0e
(cherry picked from commit 8a08a3cb47d0dd69d4aa2e8fa661d04054fe95ae)