parameterize logrotated files

Parameterization of the file list to be logrotated by /etc/logrotate.d/syslog.
This also fixes problems with different names and files for different OS families.
I meant with unique(). The type cannot be a string anyhow, since the readme states it must be an array and we use validate_array() to ensure that.

```puppet
if $logrotate_syslog == 'USE_DEFAULTS' {
  $logrotate_syslog_real = $default_logrotate_syslog
} else {
  $logrotate_syslog_real = unique($logrotate_syslog)
}
validate_array($logrotate_syslog_real)
```