Use xenial for openstack-tox-docs for all older stable branch

Currently openstack-tox-docs job use ubuntu-xenial nodeset for
stable/rocky and bionic for rest of the branches. But it seems
this job can be run on other stable/branch previous to stable/rocky
also. It is better to use xenial for all the stable branch till rocky.

openstack-tox-docs job is supposed to be run from stable/rocky on
(and build-openstack-sphinx-docs for odler branch) but this
job is also part of docs-on-readthedocs project-template so we need to
adjust the nodeset for older stable branch also.

Change-Id: Iac8a0deea04c60d0d427f7c19a29d9f165d80f26

zuul.d/jobs.yaml

@@ -648,7 +648,7 @@
       Run documentation build.
 
       Uses tox with the ``docs`` environment.
-    branches: ^(?!stable/rocky).*$
+    branches: ^(?!stable/(ocata|pike|queens|rocky)).*$
     required-projects:
       - name: openstack/requirements
     vars:
@@ -665,8 +665,9 @@
 
       Uses tox with the ``docs`` environment.
 
-      This job runs on Xenial for stable/rocky. This job is prepared
-      to make sure all stable/rocky will keep running on xenial.
+      This job runs on Xenial for stable/ocata, pike, queens and rocky. This
+      job is prepared to make sure all stable branches before stable/stein will
+      keep running on xenial.
     required-projects:
       - name: openstack/requirements
     vars:
@@ -676,6 +677,9 @@
     success-url: html/
     nodeset: ubuntu-xenial
     branches:
+      - stable/ocata
+      - stable/pike
+      - stable/queens
       - stable/rocky
 
 - job: