Make networks buildable by Hydra

[grahamc]

Users would like to be able to pre-build their networks in Hydra as part of CI. This is pretty nice, but Hydra doesn't make this very easy or possible. This could be especially cool for if NixOps were to one day support building netboot images or AMIs automatically.

[cleverca22]

both me and @deepfire have done that in the past, and could link some examples