Update to openstackdocstheme 1.18.1
Update to openstackdocstheme 1.18.1

This fixes our bug links, let's use it everywhere.

Change-Id: Ica7618bf41000fb574427c482d08fb0a0f23628a

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals