atomic: extend the root filesystem to fill VG
    
    With the change to using overlay in the root filesystem [1]. We
    need to configure the cloud images to extend the root filesystem
    by default [2]. Fixes [2]
    
    [1] https://pagure.io/atomic-wg/issue/281
    [2] https://pagure.io/atomic-wg/issue/343
    
    (cherry picked from commit 97b0af5d0062c54cb6f7b94410eb3ca94e9c13b0)