Updated tox config for multi-python testing.
Updated tox config for multi-python testing.

Change-Id: Ib1f768c6783db0e63cb51dfc767ae5a31a295c76

citrix-openstack-build/pbr,WiFast/pbr,emonty/oslo-packaging,citrix-openstack-build/pbr,klmitch/pbr,varunarya10/pbr,redhat-openstack/pbr,JioCloud/pbr,WiFast/pbr,emonty/oslo-packaging,openstack-dev/pbr,redhat-openstack/pbr,openstack-dev/pbr,citrix-openstack-build/pbr,openstack-dev/pbr,JioCloud/pbr,klmitch/pbr,varunarya10/pbr