[train][goal] Define new 'octavia-v2-dsvm-scenario-ipv6-only' job

As part of Train community goal 'Support IPv6-Only Deployments and Testing'[1],
Tempest has defined the base job 'devstack-tempest-ipv6' which will
deploy services on IPv6.

This commit adds the new job 'octavia-v2-dsvm-scenario-ipv6-only'
run on gate which is derived from 'devstack-tempest-ipv6'.

Verification structure will be:
- 'devstack-IPv6' deploy the service on IPv6
- 'devstack-tempest-ipv6' run will verify the IPv6-only setting and listen address
- 'octavia-v2-dsvm-scenario-ipv6-only' will run the tests.

This scenario test will run with the following IPv6 additional settings:
1. All OpenStack endpoints will use IPv6 addressing.
2. The lb-mgmt-net will use IPv6 addresses.

Story: #2005477
Task: #35910

Co-Authored-By: Adam Harwell <flux.adam@gmail.com>
Co-Authored-By: Carlos Goncalves <cgoncalves@redhat.com>
Co-Authored-By: Michael Johnson <johnsomor@gmail.com>
Depends-On: https://review.opendev.org/665103

[1] https://governance.openstack.org/tc/goals/train/ipv6-support-and-testing.html

Change-Id: Ieb74ef2276632f04d6324f835b164df3114b2613
2 files changed