Updates for ostree-container phase
    
    This patch connects the phase into the main script, and adds other
    modifications:
    
    * The archive is now stored in the images/ subdirectory in the compose.
    * Documentation is updated to correctly mention that variant repos are
      not available.
    * Configuration for path and name of the final archive is dropped. There
      are reasonable defaults for this and there's no point in having users
      configure it.
    * The extra message for the archive is no longer sent.
    * The pungi-make-ostree utility is no longer required in the buildroot.
    
    The pungi-make-ostree utility doesn't do any significant work. It
    modifies configuration files (which can happen on the compose host), and
    it starts other processes.
    
    This patch changes the ostree-container phase to no longer need the
    script in the buildroot. Instead, the utility is called on the compose
    host to do the config manipulation and output the needed commands. Those
    are then passed into the runroot task.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>