Bugfix: Use module config_source if it is set.

Hi Steffen

I think there is a bug in config.pp. Even if the caller of the module purefetpd defines config_source, it is never used. The  code would ALWAYS use puppet:///modules/${module_name}/${pureftpd::params::config_source} as config source and never $pureftpd::config_source.

Thanks, Michael