Add group name option to hosts, and a task to deploy each group

This is for #26

I added a new option to specify a group name for each host like this:

lollypops.deployment = {
            ssh.host = "1.2.3.4";
            ssh.user = "root";
            group = "nomad-server";
          };

And now when the module generates the Taskfile, it creates a new helper task for each unique group name. For example, this is with a few servers that have the group names "nomad-server" and "nomad-client":

> nix run '.' -- --list-all
...
task: Available tasks for this project:
* all:
* nixnomadclient1-01:                       Provision host: nixnomadclient1-01
* nixnomadclient1-02:                       Provision host: nixnomadclient1-02
* nixnomadsrv1-01:                          Provision host: nixnomadsrv1-01
* nixnomadsrv1-02:                          Provision host: nixnomadsrv1-02
* nixnomadsrv1-03:                          Provision host: nixnomadsrv1-03
* nomad-client:                             Provision group: nomad-client
* nomad-server:                             Provision group: nomad-server
* nixnomadclient1-01:check-vars:

This seems to work and satisfies what I wanted in #26 since I couldn't figure out a clean way to do it without modifying the lollypops module.

I am not sure about the task name though. Right now it's just the same as the group name, but I wonder if it'd be better to prefix it with group: or all:?

pinpox / lollypops

Add group name option to hosts, and a task to deploy each group #27