Add support for groups

[Kranzes]

it would be very helpful if you could great a group of hosts that can then be deployed at once.

Nix code example:

nixinate = {
  group.production.hosts = [ "hetznix" "swordfish" ];
  group.production.buildOn = "remote"; # available options are "remote", "local" or a specific host
};

[mrVanDalo]

I use parallel to run deployments in parallel

parallel --tag nix run ".#apps.nixinate.{1}" ::: machine1 machine2 machine3

and created aliases like alias deploy-prodction= ...

[cbleslie]

Currently I use makefiles for "group" deployments like so:

deploy-nemesis: ## Deploy - Remote deploy to Nemesis
	nix run .#apps.nixinate.nemesis

deploy-hesperides: ## Deploy - Remote deploy to Hesperides
	nix run .#apps.nixinate.hesperides

deploy-all: ## Deploy - 'Parallel' remote deploy to Hesperides & Nemesis
	make deploy-hesperides &\
	make deploy-nemesis

deploy-all-slow: ## Deploy - Remote deploy in order: Hesperides & Nemesis. Good for debugging
	make deploy-hesperides
	make deploy-nemesis

Though I think I might make use of parallel instead of calls to the make file itself. Thanks @mrVanDalo !