nixus.groups: added ability to put nodes into groups

[eyJhb]

This allows for declaring groups of nodes, ie. all home computers, all servers, VPS, etc. which allows to deploy one group at a time.
Example configuration could be:

{
  groups = {
    home.members = [
        "router"
        "server1"
        "eos"
    ];
    vps.members = [
        "vps1"
        "vps2"
        "vps3"
    ];
  };
}

And then building the script using nix-build -A config.groups.vps.deployScript deploy/.

I am unsure of having defaults that should be set for each group, would be a nice addition (also not sure how to implement it best).

TODO

• Need to add descriptions