Expand template variables in routing key

[magnusbaeck]

Dependencies

None.

Feature Request

The routing key is currently configured as a static string used for all events. That's not very useful for consumers that want to subscribe to a subset of the published messages. At time of writing there's no standardized routing key schema but once eiffel-community/eiffel-sepia#9 is merged one will be defined in Sepia. To support that schema (and variations of it) the routing key configuration option should support template variables like $TYPE that will get expanded when each message is sent.

For new installations the routing key should default to whatever gets defined in Sepia but existing installations' custom settings should be retained when upgrading.