docker: check if ftype exists

When deploying with --stack-only, the playbooks aren't executed so if a
variable is processed lated, it needs to be checked if it's actually
defined or not otherwise Ansible will fail.

Change-Id: I16a2eed003b8f7fd4c775cdeead3f3b51f8aa467

tasks/docker.yml

@@ -45,6 +45,7 @@
           XFS volumes formatted using ftype=0 are incompatible
           with the docker overlayfs driver.
       when:
+        - not ansible_check_mode
         - ftype.stdout == 'ftype=0'
 
     - name: ensure docker is installed