Remove addl template feature, improve docs

- Remove attribute for additional templates. You should now call `logstash_` LWRPs directly to get this functionality back.
- Remove automatic inclusion of `platformstack` cookbooks. These should be called by a downstream wrapper.
- Better documentation for java requirement, Berkshelf requirements, and keypair requirements for Lumberjack. Fixes https://github.com/rackspace-cookbooks/elkstack/issues/57.