Move verify-ipv6-only-deployments

from Tempest to DevStack as it tests DevStack side of things and
is useful for projects not using Tempest.

This is part 2 of 2.
The 1st part is DevStack-side, in Depends-On.

The script is left calling out to devstack because legacy (dsvm)
jobs rely on its presence.

Depends-On: Ie166730843f874b9c99e37244e460d7ad33b7eeb
Change-Id: I6fa17ae413f106453303c4882925573bd8e05029
5 files changed