Ansible>=2.3.0 will output warnings if `when` statements contain jinja2 templating delimiters such as curly braces (the playbooks are executing fine, despite the warnings).
A patch has been pushed to the upstream openstack gerrit by Florian. Just created the bug here for tracking potential backport.
Added a backport patch:
Verified in: openstack-tripleo-validations-8.4.0-2.el7ost.noarch
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.