users: Do not clear supplemental groups

To ensure the `users.yml` playbook is idempotent in cases where the
users it manages are also managed by other playbooks, we have to set
`append: true`.  This prevents the managed user(s) from being removed
from additional groups other playbooks may have added them to.

users.yml

@@ -16,6 +16,7 @@
       comment: '{{ item.comment | d(omit) }}'
       uid: '{{ item.uid | d(omit) }}'
       groups: '{{ item.groups | d(omit) }}'
+      append: true
       create_home: true
       local: true
       password: '*'