Set default value of $gc_log_file in broker.pp, don't use params.pp
Do not support multiple broker instances per machine anymore.

The functionality of the defined type broker.pp was split into
install.pp, config.pp (new file), and service.pp.

On the positive side:
This change greatly simplifies how you need to define Hiera
configuration for this module.

On the negative side:
This change breaks backwards compatiblity.  However, the
multi-broker-per-machine scenario was only ever useful for functional
testing, hence this change does not effect production deployments.