Add hiera-friendly option to manage dropins

[ondrejv2]

As per recently closed #420 we have a nice option to manage user slices etc, but the only way to make real use of it is via different module.
I would like to use this capability via Hiera - i.e. the way I'd expect it to work is something like this:

systemd::dropin_files:
  'user-limits.conf':
    unit: 'user-.slice'
    slice_entry:
        'MemoryMax': '17G'

but this obviously does not work as systemd::dropin_files does not load systemd::manage_dropin class and even if it did, the class does not create resources from hiera.

This is a friendly request improve the code to make it overall more hiera-friendly.

[ondrejv2]

Hello, just wondering when is a new release planned? Currently we have 6.6.0 which does not include fix for this request. Thanks!

[ekohl]

In #437 there was some debate about the next release.