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: I95015c79049633ed97714d6de8dd8f231bd15a03

openstack/python-swiftclient,jeseem/python-swiftclient,jeseem/python-swiftclient,sohonetlabs/python-swiftclient,openstack/python-swiftclient,sohonetlabs/python-swiftclient