Generate a Synapse signing key file, if missing

The code used to check for a `homeserver.yaml` file and generate
a configuration (+ key) only if such a configuration file didn't exist.

Certain rare cases (setting up with one server name and then
changing to another) lead to `homeserver.yaml` being there,
but a `matrix.DOMAIN.signing.key` file missing (because the domain
changed).
A new signing key file would never get generated, because `homeserver.yaml`'s
existence used to be (incorrectly) satisfactory for us.

From now on, we don't mix things up like that.
We don't care about `homeserver.yaml` anymore, but rather
about the actual signing key.

The rest of the configuration (`homeserver.yaml` and
`matrix.DOMAIN.log.config`) is rebuilt by us in any case, so whether
it exists or not is irrelevant and doesn't need checking.

roles/matrix-synapse/tasks/setup_synapse_main.yml

@@ -22,17 +22,19 @@
   docker_image:
     name: "{{ matrix_synapse_docker_image }}"
 
-- name: Check if a Matrix Synapse configuration exists
+- name: Check if a Synapse signing key exists
   stat:
-    path: "{{ matrix_synapse_config_dir_path }}/homeserver.yaml"
-  register: matrix_synapse_config_stat
+    path: "{{ matrix_synapse_config_dir_path }}/{{ matrix_server_fqn_matrix }}.signing.key"
+  register: matrix_synapse_signing_key_stat
 
-# We do this mostly so that the keys would get generated.
-# We'll replace the rest of the configuration with our own templates below.
+# We do this so that the signing key would get generated.
+#
+# This will also generate a default homeserver.yaml configuration file and a log configuration file.
+# We don't care about those configuraiton files, as we replace them with our own anyway (see below).
 #
 # We don't use the `docker_container` module, because using it with `cap_drop` requires
 # a very recent version, which is not available for a lot of people yet.
-- name: Generate initial Matrix config
+- name: Generate initial Matrix config and signing key
   command: |
     docker run
     --rm
@@ -45,7 +47,7 @@
     -e SYNAPSE_REPORT_STATS=no
     {{ matrix_synapse_docker_image }}
     generate
-  when: "not matrix_synapse_config_stat.stat.exists"
+  when: "not matrix_synapse_signing_key_stat.stat.exists"
 
 - name: Ensure Matrix homeserver config installed
   template: