Add option to not generate a default changelog entry
    
    For Rust packages created through the OBS source service, the
    changelog is managed by an external <pkg>.changes file that is
    transformed into an RPM changelog and appended to the spec file
    automatically by the Open Build Service.
    
    Having a default entry means that the changelog handling will not
    always work correctly and package builds may fail due to changelog
    entries not being in reverse chronological order.
    
    This also is a future enabler for other workflows for external
    changelog management in automated package builds.
    
    Signed-off-by: Neal Gompa <ngompa13@gmail.com>