Added support for 'userlist_deny' option.

Hi,

I added support for the _userlist_deny_ option of vsftpd.

Changes:
- Added `userlist_deny` option to __vsftpd.conf__ template.
- Added `userlist_deny` parameter to vsftpd class. The default value for the parameter is `YES`, as it is in vsftpd (so this change shouldn't affect existing installations).

I'd appreciate it if a new version with this change can be submitted to Puppet Forge.

Regards,

Dennis