Allow Synapse worker list to be generated dynamically

This leads to much easier management and potential safety
features (validation). In the future, we could try to avoid port
conflicts as well, but it didn't seem worth the effort to do it now.
Our port ranges seem large enough.

This can also pave the way for a "presets" feature
(similar to `matrix_nginx_proxy_ssl_presets`) which makes it even easier
for people to configure worker counts.

roles/matrix-synapse/tasks/validate_config.yml

@@ -12,6 +12,15 @@
     - "matrix_synapse_database_password"
     - "matrix_synapse_database_database"
 
+- name: Fail if asking for more than 1 instance of single-instance workers
+  fail:
+    msg: >-
+      `{{ item }}` cannot be more than 1. This is a single-instance worker.
+  when: "vars[item] > 1"
+  with_items:
+    - "matrix_synapse_workers_appservice_workers_count"
+    - "matrix_synapse_workers_pusher_workers_count"
+
 - name: (Deprecation) Catch and report renamed settings
   fail:
     msg: >-