Add Pantalaimon support

This is actually authored by Julian Foad here
(https://lab.trax.im/matrix/matrix-docker-ansible-deploy), but was in
need of a rebase and various adjustments caused by huge playbook
refactoring that landed in the past months.

This rework is completely untested.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/266

roles/custom/matrix-pantalaimon/tasks/validate_config.yml

@@ -0,0 +1,8 @@
+---
+
+- name: Fail if required variables are undefined
+  ansible.builtin.fail:
+    msg: "The `{{ item }}` variable must be defined and have a non-null value."
+  with_items:
+    - "matrix_pantalaimon_homeserver_url"
+  when: "vars[item] == '' or vars[item] is none"