atomic: switch to new tag for installer
    
    Switch to the f27-atomic-host-installer tag. This tag takes the
    place of the f27-atomic tag and is used to tag new versions of
    packages that we want in the installer ISO for Atomic Host.
    
    See https://pagure.io/releng/issue/7100
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>