python 2.6 require stevedore>=1.8.0,<=1.10.0
    but,there is no constraint condition in setup.py,
    it will cause vt-bootstrap failed with python 2.6.
    So define the stevedore version conditionally on
    the current python version

issue:
    https://github.com/avocado-framework/avocado/issues/1613Signed-off-by: NYanan Fu <yfu@redhat.com>