DCN Multistack support for FFU

The DCN topology consists of multiple stacks which may not
include controller nodes (only compute/ceph nodes). Each such
stack is upgraded separately and no controllers are valid result.
Let's not fail If there are no controllers and move on with no
controllers.

Change-Id: Iaf08bc5aee4b97ab630e37ac7c49fdf6c316d8ea

tasks/common/controller_post_scripts.yml

@@ -2,4 +2,4 @@
 - include_tasks: controller_post_script.yml
   loop_control:
     loop_var: node_name
-  loop: "{{ inventory_hostmap[controller_role_name] }}"
+  loop: "{{ inventory_hostmap[controller_role_name] | default([]) }}"

tasks/fast-forward-upgrade/overcloud_upgrade_fast_and_furious.yaml

@@ -1,13 +1,16 @@
 ---
 - set_fact:
     pcs_host: ""
+- set_fact:
+    boostrap_node: "{{ ((oc_roles_hosts|dict2items |
+                      selectattr('key', 'in', inventory_rolemap['mysql'] | default([]))) |
+                       map(attribute='value') | map('sort') |  list | flatten(1))[0] | default('') }}"
 - name: Create /var/lib/tripleo/transfer-flags/var-lib-mysql in bootstrap node
   become: true
   become_user: "{{ (overcloud_ssh_user) | ternary(overcloud_ssh_user, 'heat-admin') }}"
-  delegate_to: "{{ ((oc_roles_hosts|dict2items |
-                     selectattr('key', 'in', inventory_rolemap['mysql'] | default([]))) |
-                     map(attribute='value') | map('sort') |  list | flatten(1))[0]   }}"
+  delegate_to: "{{ boostrap_node }}"
   shell: "sudo mkdir -p /var/lib/tripleo/transfer-flags && sudo touch /var/lib/tripleo/transfer-flags/var-lib-mysql"
+  when: boostrap_node|bool
 
 - name: upgrade the whole overcloud
   vars: