tempest does not fully support ipv6

There is currently no way to run tempest with tenant isolation in an environment that only uses ipv6. There should be an option to control whether the isolated tenant code creates subnets with ipv4, ipv6, or both.

In addition, there are places in the code that do get_server(id)['addresses'][some-network-name][0] and then do something with the subnet descriptor. There was only one place in the code that was failing (due to the need to use 'ip6' instead of 'ip' for list server filter), but because the subnet list seems to be non-deterministic in terms of ordering, there may be subtle bugs in the other usages that can be found by searching for "'addresses'".