Remove interface json and set skip_test method

According to last comments to previous patch:
https://review.openstack.org/#/c/117458/28
 - remove interface=json
 - add skip checks to separate method

Change-Id: Id596b8d33533088f79e20293a980ad2ec80198a6
Merge "Remove interface json and set skip_test method"
Merge branch 'master' of https://github.com/openstack/tempest