Backup in ISO feature

As discussed in #264, I implemented that feature by moving the output phase after the backup creation but only when it's needed to integrate the backup in the ISO. 
In this way, they won't be any regression for other output type such as ODBR.
It wasn't possible to keep the current order because it implies to much changes in output and a lot of code duplication, especially for the ISO copy to OUTPUT_URL.

As Dag suggested, I implemented a new iso scheme for BACKUP_URL which avoid a new option.
For instance, BACKUP_URL=iso://my_backups  will copy backups in a subdir called my_backups inside the ISO image.
But, to use this feature, OUTPUT_URL needs to be defined to another location.