test-playbooks: avoid warnings with shell/command

Avoids runtime warnings from use of Ansible shell/command module when
executed commands also have ansible modules.

Change-Id: I4e415cbd34f0f4cb15857051bf95458e0316de86

test-playbooks/javascripts/fetch-javascript-tarball.yaml

@@ -4,6 +4,8 @@
       shell: |
         mkdir -p {{ zuul.project.src_dir }}
         tar czf {{ zuul.project.src_dir }}/dist.tgz /etc/os-release
+      args:
+        warn: false
   tasks:
     - import_role:
         name: fetch-javascript-tarball