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: I442b41b3d9e4e536694b95a34ce981957268204e

eayunstack/fuel-web,eayunstack/fuel-web,huntxu/fuel-web,huntxu/fuel-web,stackforge/fuel-web,eayunstack/fuel-web,huntxu/fuel-web,huntxu/fuel-web,huntxu/fuel-web,eayunstack/fuel-web,eayunstack/fuel-web,stackforge/fuel-web,stackforge/fuel-web