Add post_upgrade_tasks with post_upgrade_steps_playbook output

This wires up the post_upgrade_tasks tasks to be written as
ansible playbooks like the upgrade/update_tasks. This will
write out a post_upgrade_steps_playbook ansible playbook.

Used in https://review.openstack.org/#/c/489201/ by ODL
and https://review.openstack.org/#/c/505603/ cinder-volume
Q upgrade spec @ Ibde21e6efae3a7d311bee526d63c5692c4e27b28
Related Blueprint: major-upgrade-workflow

Change-Id: Ib6188c91076eabf20d6e358ca247fed802369a7f

common/deploy-steps.j2

@@ -20,6 +20,7 @@
 {% set deploy_steps_max = 6 -%}
 {% set update_steps_max = 6 -%}
 {% set upgrade_steps_max = 6 -%}
+{% set post_upgrade_steps_max = 2 -%}
 
 heat_template_version: queens
 
@@ -553,3 +554,15 @@ outputs:
               with_sequence: start=0 end={{upgrade_steps_max-1}}
               loop_control:
                 loop_var: step
+      post_upgrade_steps_tasks: |
+{%- for role in roles %}
+            - include: {{role.name}}/post_upgrade_tasks.yaml
+              when: role_name == '{{role.name}}'
+{%- endfor %}
+      post_upgrade_steps_playbook: |
+        - hosts: overcloud
+          tasks:
+            - include: post_upgrade_steps_tasks.yaml
+              with_sequence: start=0 end={{post_upgrade_steps_max-1}}
+              loop_control:
+                loop_var: step