`systemd::manage_unit` creates conflicting parameter set when `enable` is set to `mask`

[lelutin]

As the title says, the systemd::manage_unit defined type advertises the possibility of setting the ensure parameter to a value of mask but that fails puppet runs.

A simple reproducing case:

systemd::manage_unit { 'tmpfs.mount':
  enable => 'mask',
}

The above produces this catalog compilation error:

Error: Failed to apply catalog: Validation of File[/etc/systemd/system/tmpfs.mount] failed: You cannot specify more than one of content, source, target (file: /etc/puppet/code/environments/feature_branch_name/modules/systemd/manifests/unit_file.pp, line: 105)

This is caused by the fact that manage_unit directly delegates the value of enable to systemd::unit_file but also unconditionally sets the content param for that same resource type.