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: I5c2fedbdb26f2617b99cc15c3f82c89263745977

openstack/ceilometer-powervm,openstack/ceilometer-powervm,stackforge/ceilometer-powervm,stackforge/ceilometer-powervm