Remove use of tox-siblings role
Remove use of tox-siblings role

Its functionality has been merged into the tox role, so is no longer
needed.

Depends-On: Id61ae52d48b28cfc2221cb556a1c1f7c6dfd60dd
Change-Id: Iec14ffd5d6bf7bd3faefd250b0cf2f1563ec3521

openstack/tacker,stackforge/tacker,openstack/tacker,openstack/tacker,stackforge/tacker