Fix xtrabackup regression introduced in #1207

[fraenki]

Currently the mysql::backup::xtrabackup class is nonfunctional:

Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Function Call, 
Class[Mysql::Backup::Xtrabackup]: has no parameter named 'backup_success_file_path' 
(file: /etc/puppetlabs/code/environments/development/modules/mysql/manifests/server/backup.pp, line: 98, column: 3) 

This is due to an oversight in #1207, it was forgotten to add the parameter $backup_success_file_path to the manifest (but it was added to the template).

This PR adds the missing parameter $backup_success_file_path.

[fraenki]

The failed test...

Error: Execution of '/usr/bin/mysql --database=mysql -e SET PASSWORD FOR 'root'@'localhost' = '*2470C0C06DEE42FD1618BB99005ADCA2EC9D1E19'' returned 1:

...is NOT caused by this PR :)