Improve Octavia AZ/DCN support

• Allow the user to create an Active-Standby load balancer with Amphora VMs in 2 different AZs
• the VIP network should be stretched between the AZs (due to VRRP)
• when an AZ goes down, the Amphora on the 2nd AZ becomes the Active amphora

Proposed spec upstream: https://review.opendev.org/c/openstack/octavia/+/923571