Switch to nixops and/or terraform

[rvolosatovs]

As our infrastructure grows, current deployment tool does not seem enough.
https://github.com/NixOS/nixops was undergoing some development for transition to flakes, but it should be in a more stable state now, so we should switch to it.

We should also investigate if Terraform, which is very well supported by NixOS is something that can help us here.