atomic: don't use cmdline for install mode
    
    similar change was done for docker/cloud in f6ecdc3
    
    cmdline makes it so that %post --erroronfail won't actually stop the
    installation in a way that imagefactory will detect the problem and
    fail the build. See [1] for more details.
    
    [1] https://github.com/rhinstaller/anaconda/issues/931