Allow to specify $CFG->backupdir instead of default $CFG->tempdir . '/backup'

Ideally, any component/area touched by the changes in the PR should be tested i.e. backuping and restoring courses, including blocks, resources and activities, even Moodle 1 backups. Regressions A new Moodle instance installation (done both via WEB and CLI) Unit and Behat tests should pass Restore a course on it Create a new course with some activities, create a backup of it and download the backup locally on your PC, then delete the course and restore it by means of the last backup An existing Moodle instance, upgraded Unit and Behat tests should pass Restore a course on it A new Moodle instance installation done via CLI with a custom dataroot (- MDL-30381 -) Unit and Behat tests should pass Restore a course on it Create a new course with some activities, create a backup of it and download the backup locally on your PC, then delete the course and restore it by means of the last backup Configure a different $CFG->backuptempdir in a new installation initially using the default path Repeat the tests above for a new Moodle installation, looking now at the contents of the custom backup temp directory that should contain some files, including the .htaccess file touch the temp backup files to change the modified attribute to get them older than 1 week (e.g. touch -md "1 Feb" <file> ), then force the run of the Delete stale temp files : the "touched" temp backup files should be removed since they are older than 1 week