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: I9d3a8051ed554e8400f2da0d11390bab194af34c

bac/horizon,openstack/horizon,django-leonardo/horizon,davidcusatis/horizon,yeming233/horizon,bac/horizon,redhat-openstack/horizon,NeCTAR-RC/horizon,NeCTAR-RC/horizon,sandvine/horizon,coreycb/horizon,redhat-cip/horizon,Mirantis/mos-horizon,gerrive/horizon,redhat-openstack/horizon,dan1/horizon-x509,coreycb/horizon,coreycb/horizon,Mirantis/mos-horizon,sandvine/horizon,davidcusatis/horizon,sandvine/horizon,Tesora/tesora-horizon,noironetworks/horizon,Tesora/tesora-horizon,BiznetGIO/horizon,redhat-cip/horizon,BiznetGIO/horizon,bac/horizon,wolverineav/horizon,openstack/horizon,yeming233/horizon,coreycb/horizon,redhat-openstack/horizon,Mirantis/mos-horizon,Mirantis/mos-horizon,ChameleonCloud/horizon,yeming233/horizon,BiznetGIO/horizon,Tesora/tesora-horizon,gerrive/horizon,davidcusatis/horizon,redhat-cip/horizon,redhat-cip/horizon,openstack/horizon,redhat-openstack/horizon,bigswitch/horizon,dan1/horizon-x509,dan1/horizon-x509,Tesora/tesora-horizon,wolverineav/horizon,gerrive/horizon,noironetworks/horizon,bigswitch/horizon,wolverineav/horizon,dan1/horizon-x509,ChameleonCloud/horizon,bigswitch/horizon,gerrive/horizon,django-leonardo/horizon,ChameleonCloud/horizon,django-leonardo/horizon,wolverineav/horizon,openstack/horizon,django-leonardo/horizon,noironetworks/horizon,noironetworks/horizon,bac/horizon,NeCTAR-RC/horizon,davidcusatis/horizon,BiznetGIO/horizon,sandvine/horizon,ChameleonCloud/horizon,bigswitch/horizon,yeming233/horizon,NeCTAR-RC/horizon