Transform shell/ls task to use "find" module

The Ansible way to list files which match a certain parttern is to use
the find module:

https://docs.ansible.com/ansible/latest/modules/find_module.html

Also adding a comment that the task isn't useful for now, and modify a
comment which recommends to use with_items (deprecated) while a loop
should be used with newer Ansible.

Change-Id: I67e55857d7d510d96319ee767473c11942b79239

roles/validate-yaml/tasks/main.yaml

@@ -18,14 +18,16 @@
 
   # TODO(arxcruz) Right now there are yaml files that are not
   # a valid tempest-skiplist file, so we are using just one for now
-  - name: List all yaml files to be tested
-    shell: ls roles/validate-tempest/vars/*.yml
-    args:
-      chdir: "{{ tempest_skip_path }}"
+  # The task isn't useful until we get rid of the old yaml files
+  # (see next task).
+  - name: Find all yaml files to be tested
+    find:
+      paths: "{{ tempest_skip_path }}/roles/validate-tempest/vars"
+      patterns: '*.yml,*.yaml'
     register: yaml_files
 
   # TODO(arxcruz) Once we get rid of the old yaml files,
-  # use yaml_files.stdout_lines in a with_items
+  # use yaml_files.files in a loop
   - name: Validate tempest-skip yaml files
     shell:
       set -ex
@@ -36,4 +38,4 @@
       chdir: "{{ tempest_skip_path }}"
 
   vars:
-    tempest_skip_path: "{{ zuul.projects['opendev.org/openstack/openstack-tempest-skiplist'].src_dir }}"
+    tempest_skip_path: "{{ zuul.projects['opendev.org/openstack/openstack-tempest-skiplist'].src_dir }}"