Problems if mysql.sock is not in /tmp folder

The xtrabackup doesn't find the sock file if it's not in the /tmp folder (what might happen depending on the mysql service file..)

It might be necessary to provide the socket information to xtrabackup but the scripts don't allow that. So an attribute like --socket= would be awesome!

I've solved the problem right now by putting a cnf file in /etc/mysql/conf.d/.
