Prevent bootstrap failure when all roles/collections are overriden

At the moment we assume that scenario when all roles or collections
provided by ansible-role/collection-requirements won't be overwritten.
As if they do, previous set_fact will be skipped and
galaxy_collections_list/clone_roles will remain undefined.

To avoid failure and to allow to fully override roles/collections we
define default for the task that aims to merge default items with
user-provided ones.

Change-Id: I86d44499073f0571a529d81ed3be713996e5f339

scripts/get-ansible-collection-requirements.yml

@@ -42,7 +42,7 @@
 
     - name: Append user collections to filtered required collections
       set_fact:
-        galaxy_collections_list: "{{ galaxy_collections_list + [ item ] }}"
+        galaxy_collections_list: "{{ (galaxy_collections_list | default([])) + [ item ] }}"
       with_items: "{{ user_collections.collections }}"
       when:
         - user_collections.collections is defined

scripts/get-ansible-role-requirements.yml

@@ -132,7 +132,7 @@
 
     - name: Append user overridden roles
       set_fact:
-        clone_roles: "{{ clone_roles + user_roles }}"
+        clone_roles: "{{ (clone_roles | default([])) + user_roles }}"
 
     - name: Clone git repos
       block: