Webservice: Update Kafka config to use EuXFEL broker

The DESY Kafka brokers we were using don't seem to be available to us any longer. We've been meaning to switch over to the EuXFEL broker anyway, so this seems like a good time to do it.

Producers should be authenticated for this, so there's some new config to manage the authentication. The topic is namespaced under the username, so this will change as well.

I've asked Janusz S to set up the account and give me the password; I'll test it when I've got this. My mistake, I already had this info.

@schmidtp @jszuba