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.

fedora-common-ostree.yaml

@@ -37,7 +37,7 @@ packages:
 
 selinux: true
 documentation: true
-boot_location: new
+boot-location: modules
 etc-group-members:
   - wheel
 tmp-is-dir: true