Deprecated tox -downloadcache option removed
Deprecated tox -downloadcache option removed

Caching is enabled by default from pip version 6.0

More info:
https://testrun.org/tox/latest/config.html#confval-downloadcache=path
https://pip.pypa.io/en/stable/reference/pip_install/#caching

Change-Id: I5376fa4891a1a1f469af36d850b4045d24590aec

phenoxim/cinder,cloudbase/cinder,NetApp/cinder,eharney/cinder,ge0rgi/cinder,NetApp/cinder,Nexenta/cinder,j-griffith/cinder,dims/cinder,openstack/cinder,dims/cinder,phenoxim/cinder,bswartz/cinder,Hybrid-Cloud/cinder,cloudbase/cinder,Datera/cinder,mahak/cinder,Datera/cinder,openstack/cinder,j-griffith/cinder,eharney/cinder,Nexenta/cinder,Hybrid-Cloud/cinder,mahak/cinder,bswartz/cinder