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: Id9cb6a024049932501c41b393ce9cf9ad7c2cc86

openstack/doc8,doismellburning/doc8,stackforge/doc8