Ensure matrix_dynamic_dns_domain_configurations contains configurations

If `matrix_dynamic_dns_enabled`, we'd like to ensure there's at least
one configuration defined.

Related to #681 (Github Pull Request)

roles/matrix-dynamic-dns/tasks/validate_config.yml

@@ -1,5 +1,11 @@
 ---
 
+- name: Fail if no configurations specified
+  fail:
+    msg: >-
+      You need to define at least one configuration in `matrix_dynamic_dns_domain_configurations` for using matrix-dynamic-dns.
+  when: "matrix_dynamic_dns_domain_configurations|length == 0"
+
 - name: Fail if required settings not defined in configuration blocks
   fail:
     msg: >-