tests: Fix issue with 'ovdk-vport' tests

In some cases these tests can fail due to missing environment vars.
This was because the environment wasn't carried over when executing
sudo. Add the '-E' flag to fix this.

Change-Id: Ie212a6c3de40fd2aea0b8fa8fbbd25de0b76ba59
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Kevin Traynor <kevin.traynor@intel.com>