bodhi-pungi: use AtomicHost variant for AH things
    
    The compose today failed with:
    
    ```
    [WARNING ] [OSTREE_INSTALLER] Patterns in config do not match any variant: ^Atomic$
    ```
    
    There is no Atomic variant any longer. Let's use AtomicHost instead,
    which will sync it up with what is done inthe pungi-fedora repo.