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: I955fe51b5058ba2ae659ef13718555a62bce67ec

jiahaoliang/group-based-policy,noironetworks/group-based-policy,stackforge/group-based-policy,noironetworks/group-based-policy,jiahaoliang/group-based-policy,stackforge/group-based-policy