upgrade: Download only /usr/share/rpm in check-diff

        This improves on the check-diff option by only downloading the
        /usr/share/rpm directory to do a package diff.  This prevents downloading
        the whole deployment and the necessity to do a cleanup later.

upgrade: Add check-only option to test if a new version is ready

        This option queries the ostree repo to compare the checksums of
        versions to determine if a new update is available.  This allows
        a sysadmin to check for updates without commiting to deploying
        the new version.  Future iterations could include a package diff
        instead of a simple true/false.