Use case: multi-nic ipv6-only launch

In cloud-init, we'd like to integration test scenarios that involve ipv6-only subnets and multinic already setup on first boot. We first need the plumbing in pycloudlib to make it happen.

Start by tracing pycloudlib's existing instance-launch plumbing and the integration-test setup used for cloud-init scenarios. Identify how first-boot networking is configured for multiple interfaces, then define what is needed to launch an IPv6-only, multi-NIC scenario and verify that the integration test can exercise it.

Written by the indexing model from the issue text.