atomic: use basearch in ostreesetup line
    
    This will help enable us to use the same kickstart file for multiple
    architectures. Enabling work was in the pull requests listed below:
    
    https://github.com/projectatomic/rpm-ostree/pull/877
    https://github.com/rhinstaller/anaconda/pull/1186
    
    (cherry picked from commit c3095c82baaa12e24131b27be9c321edf39bf05f)