Pin doc tox build to stable/pike
Pin doc tox build to stable/pike

Change to neutron/master required this change.

Change-Id: Ib212426320f01024ea2f4077acd98d1b98531424

openstack/networking-cisco,noironetworks/networking-cisco,Tehsmash/networking-cisco,noironetworks/networking-cisco,openstack/networking-cisco,Tehsmash/networking-cisco