gate_hook: Add "tempest" case
gate_hook: Add "tempest" case

Will be used by gate-neutron-dsvm-tempest-vpnaas-ubuntu-xenial.

Partial-Bug: #1659691
Change-Id: I81e712503a20623bf43bd61c39f79efc306f8e9a

openstack/neutron-vpnaas,openstack/neutron-vpnaas