Changes to allow users of cookbook to implement custom logic to restart

Changes in this PR will let users of the cookbook to be able to implement restart logic for Kafka service in their wrapper cookbook. Please refer request in issue #58 .

Note that the changes are made in v0.6.0 of the cookbook.
Would prefer if this section was above the `default.kafka.broker = {}` one (also with a newline above the top-most comment character).