using hiera_hash for variable lookup

I've made a small change to way how `users` and `groups` are looked up. However it might have a huge impact, from now on, `ssh_users` variable declaration will be merged from all relevant files. So, you can have "super" users available on all machines and then other users having accounts just on a single machine.

It might result in unexpected behaviour for someone who was using previous versions, because not just first occurrence of `ssh_users` is used, but all of them. Issuing a new version might be good idea.