Move verify-ipv6-only-deployments

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

Verbatim copy except for the devstack- prefix and the /devstack/
path.

Change-Id: Ie166730843f874b9c99e37244e460d7ad33b7eeb
diff --git a/roles/devstack-ipv6-only-deployments-verification/defaults/main.yaml b/roles/devstack-ipv6-only-deployments-verification/defaults/main.yaml
new file mode 100644
index 0000000..fea05c8
--- /dev/null
+++ b/roles/devstack-ipv6-only-deployments-verification/defaults/main.yaml
@@ -0,0 +1 @@
+devstack_base_dir: /opt/stack