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: I3cb2252fbb3fbb5e80a46bd0fa18eadabba0c3ee

stackforge/refstack-client,openstack/refstack-client,openstack/refstack-client,stackforge/refstack-client