Make it possible to target workers with IPv4 and/or IPv6 connectivity

There are some autopkgtests that rely on interacting with services that are only available over IPv4, it would be nice if we could use a combination of task configuration and worker tags (#326) to make sure that the tests run on workers with IPv4 connectivity.

We should obviously do the same for IPv6 too, and thus also make it possible to require a working dual-stack by using the two tags together.