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: I827a7c2b9955cd84d96cba2f4b860f0977d541b1

openstack/os-brick,openstack/os-brick