Get nixos test containers building in Hydra

[jfly]

The new nixos test container infrastructure requires a few tweaks to our machines:

• https://github.com/NixOS/nixpkgs/blob/master/nixos/doc/manual/development/running-nixos-tests.section.md#system-requirements-sec-running-nixos-tests-requirements
• nixos/doc: note that /dev/net is required for nspawn↔qemu networking nixpkgs#503006

We ran into issues in the past with cgroups: NixOS/nix#13135

We are unsure if it's safe to expose /dev/net in the build sandbox. This is only required for nspawn <-> qemu communication, so we could disable just those tests (nixosTests.nixos-test-driver.containers) while still allow for tests that are purely nspawn containers (such as nixosTests.test-containers-bittorrent).

[jfly]

FYI, @kmein @tfc. Tests that rely upon /dev/net are currently disabled in hydra. I've added this to the infra team's list of topics to discuss this week. Edit: busy meeting this week, we will discuss next time.