fedora-common-ostree: Switch to modules for boot-location
    
    Fixes the following warning:
    warning: boot-location: "new" is deprecated, use boot-location: modules
    
    From the rpm-ostree docs (https://coreos.github.io/rpm-ostree/treefile/):
    
      - “new”: A misnomer, this value is no longer “new”. Kernel data goes
        in /usr/lib/ostree-boot in addition to /usr/lib/modules. This is the
        default; use it if you have a need to care about upgrading from very
        old versions of libostree.
    
      - “modules”: Kernel data goes just in /usr/lib/modules. Use this for
        new systems, and systems that don’t need to be upgraded from very
        old libostree versions.