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: I3d9cf319804532b2b16d14cbc5ba2621c9b082de

openstack/tripleo-puppet-elements,openstack/tripleo-puppet-elements