Make xtrabackup binary create backup-my.cnf and use it by default

Currently when multiple datafiles are used with innodb_data_file_path, it is important to use the same configuration on --prepare. However, xtrabackup would use the default one (and fail) unless backup-my.cnf created by innobackupex is explicitly specified at --prepare.

There's also no backup-my.cnf if the backup was created with just xtrabackup binary without innobackupex.

The solution would be to make xtrabackup (rather innobackupex as currently) create backup-my.cnf, and then use it by default when preparing a backup