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: I4d0b1432e5e8e60bca06fa094a99946523155af6

openstack/python-opentsdbclient,stackforge/python-opentsdbclient