use concat to manage /etc/foreman/settings.yaml

Foreman plugins usually stores configuration in foreman's settings.yaml, so we can't manage settings.yaml as a single File resource.

With this patch, I can do that to configure, for example, column_view plugin:

```puppet
concat::fragment {'foreman_settings_column_view':
  target  => '/etc/foreman/settings.yaml',
  content => ':column_view:
  :title: Uptime
  :content: facts_hash[\'uptime\']
',
  order   => 10,
}
```