Make /.well-known/matrix/server optional

People who wish to rely on SRV records can prevent
the `/.well-known/matrix/server` file from being generated
(and thus, served.. which causes trouble).

roles/matrix-nginx-proxy/tasks/self_check_well_known.yml

@@ -1,13 +1,26 @@
 ---
 
+- name: Determine well-known files to check (Matrix)
+  set_fact:
+    well_known_file_checks:
+      - path: /.well-known/matrix/client
+        purpose: Client Discovery
+        cors: true
+
+- block:
+    - set_fact:
+        well_known_file_check_matrix_server:
+          path: /.well-known/matrix/server
+          purpose: Server Discovery
+          cors: false
+
+    - name: Determine domains that we require certificates for (mxisd)
+      set_fact:
+        well_known_file_checks: "{{ well_known_file_checks + [well_known_file_check_matrix_server] }}"
+  when: "matrix_well_known_matrix_server_enabled"
+
 - name: Perform well-known checks
   include_tasks: "{{ role_path }}/tasks/self_check_well_known_file.yml"
-  with_items:
-    - path: /.well-known/matrix/server
-      purpose: Server Discovery
-      cors: false
-    - path: /.well-known/matrix/client
-      purpose: Client Discovery
-      cors: true
+  with_items: "{{ well_known_file_checks }}"
   loop_control:
     loop_var: well_known_file_check