Run tox without need of sitepackages
    
    * Since koji-1.15, it supports setuptools. There is no need of koji
      installed in system.
    * So far, koji-1.15 has not been published to PyPI. As a result, koji
      is installed from source code from tag koji-1.15.0.
    * tox.ini is updated to run without the need of option sitepackages.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>