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)