Ensure Discord client id is passed as a string

Looks like these client ids are actually integers,
but unless we pass them as a string, the bridge would complain with
an error like:

    {"field":"data.auth.clientID","message":"is the wrong type","value":123456789012345678,"type":"string","schemaPath":["properties","auth","properties","clientID"]}

Explicitly-casting to a string should fix the problem.

The Discord bridge should probably be improved to handle both ints and
strings though.

roles/matrix-bridge-appservice-discord/defaults/main.yml

@@ -69,7 +69,7 @@ matrix_appservice_discord_configuration_yaml: |
     disableJoinLeaveNotifications: false
   # Authentication configuration for the discord bot.
   auth:
-    clientID: {{ matrix_appservice_discord_client_id }}
+    clientID: {{ matrix_appservice_discord_client_id|string }}
     botToken: {{ matrix_appservice_discord_bot_token }}
   logging:
     # What level should the logger output to the console at.