This moves the diff-and-activate logic from cluster/nix/provision.nix
into ops/{provision,machines}.nix that can be used for both cluster
machines and bgpwtf machines.
The provisioning scripts now live per-NixOS-config, and anything under
ops.machines.$fqdn now has a .passthru.hscloud.provision derivation
which is that script. When ran, it will attempt to deploy onto the
target machine.
There's also a top-level tool at `ops.provision` which builds all
configurations / machines and can be called with the machine name/fqdn
to call the corresponding provisioner script.
clustercfg is changed to use the new provisioning logic.
Change-Id: I258abce9e8e3db42af35af102f32ab7963046353
