Made the $databases parameter in mysqldump::backup an array

Previously the $databases parameter was a space-separated string and the
resulting dump files had generic names. The dump name is now generated from the
$databases parameter, which allows using several mysqldump::backup defines on
one node without unwanted side-effects. This new approach is the same as that
used in the xtrabackup module.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>