spec: simplify docker-storage-setup install
    
    Now that all of the platforms default to overlay2 on fresh installs, we
    can simplify the spec file a lot wrt. docker-storage-setup.
    
    Note the 'STORAGE_DRIVER=overlay2' is already injected at %install time
    by `make install-docker`.
    
    This should work fine when upgrading installs with devicemapper since
    c-s-s now exits nicely if /etc/sysconfig/docker-storage already exists
    and specifies a different driver.
    
    The motivation for this patch is the fact that the `%posttrans` is not
    guaranteed to be run after `/etc/os-release` is ready depending on the
    RPM transaction ordering. See also:
    https://github.com/coreos/fedora-coreos-tracker/issues/10
    
    Based originally on a patch from:
    Colin Walters <walters@verbum.org>