Adding FileLogger object. #7229.

https://dev.icinga.org/issues/7229
You'll have to set `$target_file_name` to `$name`. The `$name` variable is available and defined when Puppet evaluates the defined type, but it looks like the `$object_name` variable doesn't get 'set' internally until after it tries to create the file resource. When I tested it, it created the file as `/etc/icinga2/conf.d/.conf`.