Migrate openstack-tox-docs jobs to Ubuntu Focal

As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).

openstack-tox-docs jobs is migrated to ubuntu focal for victoria
onwards and bionic from stable/stein till stable/ussuri and xenial
till stable/rocky.

Also add texlive-full to be installed for pdf doc building otherwise
it fails on Focal:
- https://zuul.opendev.org/t/openstack/build/72a590901f43415c93f5352363561de6

[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal.html

Change-Id: Ifeeced1a5f54f1de28c89cc7d1fcd3b2ab343354

roles/prepare-build-pdf-docs/tasks/main.yaml

@@ -8,6 +8,7 @@
   package:
     name:
       - latexmk
+      - texlive-full
       - texlive-xetex
       - texlive-fonts-recommended
       - fonts-liberation

zuul.d/jobs.yaml

@@ -572,6 +572,7 @@
 - job:
     name: openstack-tox-docs
     parent: opendev-tox-docs
+    nodeset: ubuntu-focal
     description: |
       Run documentation build.
 
@@ -592,7 +593,7 @@
          file is build if the tox environment specified by
          :zuul:jobvar:`tox_pdf_envlist` exists.
 
-    branches: ^(?!stable/(ocata|pike|queens|rocky)).*$
+    branches: ^(?!stable/(ocata|pike|queens|rocky|stein|train|ussuri)).*$
     required-projects:
       - name: openstack/requirements
     vars:
@@ -607,6 +608,27 @@
     pre-run: playbooks/tox-docs/pre.yaml
     run: playbooks/tox-docs/run.yaml
 
+- job:
+    name: openstack-tox-docs
+    parent: opendev-tox-docs
+    description: |
+      Run documentation build (bionic).
+
+      Uses tox with the ``docs`` environment.
+
+      This job runs on bionic for stable/stein, train, and ussuri.
+    required-projects:
+      - name: openstack/requirements
+    vars:
+      tox_constraints_file: '{{ ansible_user_dir }}/src/opendev.org/openstack/requirements/upper-constraints.txt'
+      tox_envlist: docs
+      bindep_profile: compile doc
+    nodeset: ubuntu-bionic
+    branches:
+      - stable/stein
+      - stable/train
+      - stable/ussuri
+
 - job:
     name: openstack-tox-docs
     parent: opendev-tox-docs