Build a config file with a supplied list of file paths, indicating any that are missing

files_of_interest.j2

{# If there were any files missing on the remote host then remove them from #}
{# the files_of_interest list using the "difference" filter. #}
{% if missing_files_of_interest | length > 0 %}
{% set files_of_interest = files_of_interest | difference(missing_files_of_interest) %}
############################################################################
# The following files were configured but don't exist on this host right now:        
{% for file in missing_files_of_interest %}
# {{ file }}
{% endfor %}
############################################################################

{% endif %}
{% for file in files_of_interest %}
{{ file }}
{% endfor %}

foi.yml

---
- hosts: all 
  vars:
      files_of_interest:
          - /var/log/syslog
          - /var/log/auth.log
          - /does/not/exist
  tasks:
      - name: Check that all of the interesting files actually exist
        stat:
            path: "{{ item }}"
        with_items: "{{ files_of_interest }}"
        register: check_files_of_interest

      - name: Initialize missing file list                                           
        set_fact:
            missing_files_of_interest: []

      - name: Build list of missing files
        set_fact:
            missing_files_of_interest: "{{ missing_files_of_interest + [ item.item ] }}"
        with_items: "{{ check_files_of_interest.results }}"
        when: item.stat.exists == False

      - name: Build /etc/foo/files_of_interest
        template:
            src: files_of_interest.j2
            dest: /etc/foo/files_of_interest