allow the rabbitmq routing key to be populated from the event

The jruby implementation of the rabbitmq plugin currently doesn't do string interpolation on the routing key.

The ruby (bunny) implementation does and the tutorial on rabbitmq (http://logstash.net/docs/1.4.0/tutorials/just-enough-rabbitmq-for-logstash) indicates the same.

This PR fixes that. There may be a more elegant want to do this, but this seemed pretty straight forward. I also am not sure how this works in terms of thread safety. 

This may not be the right answer, but being able to interpolate fields in the routing key is a very nifty little feature.