Add support for more flexible container port exposing

Fixes #171 (Github Issue).

roles/matrix-bridge-appservice-discord/tasks/validate_config.yml

@@ -8,3 +8,12 @@
   with_items:
     - "matrix_appservice_discord_client_id"
     - "matrix_appservice_discord_bot_token"
+
+- name: (Deprecation) Catch and report renamed appservice-discord variables
+  fail:
+    msg: >-
+      Your configuration contains a variable, which now has a different name.
+      Please change your configuration to rename the variable (`{{ item.old }}` -> `{{ item.new }}`).
+  when: "item.old in vars"
+  with_items:
+    - {'old': 'matrix_appservice_discord_container_expose_client_server_api_port', 'new': '<superseded by matrix_appservice_discord_container_http_host_bind_port>'}