Change all `configuration` attributes to `nil`

Kafka does a pretty good job at keeping track of default values on it's
own, and I see no point in attempting to keep default values consistent
among different versions of Kafka. That's up to them to manage.

Everything is still highly configurable, but with the sane defaults that
the Kafka team decides on.
Consider `empty?` values as not being configured

Also make sure to #join `log.dirs` when passing 'em to the `option`
partial. Not sure how this has even worked before though...
Update `kafka_jar_path` helper to account for v0.8.1

Because things are packaged differently, the jar is in the `libs`
directory instead of in the root directory (of the package that is).
Remove unnecessary attributes from .kitchen.yml
Set port in default configuration

Otherwise the checks that are defined in init scripts won't work
correctly as they check if *something* is listening on a specific
port.
Add `background_threads` attribute

Only used for kafka > v0.8.0.
Ignore `per_topic` configuration attributes for kafka > v0.8.0

They're all dynamic configuration options nowadays.
Add leader & offset management related configuration attributes

Only for kafka > v0.8.0.
Add `log cleaner` configuration attributes

Only for kafka > v0.8.0.
Additional `log` configuration attributes

* `delete_delay_ms`.
* `flush_offset_checkpoint_interval_ms`.

Only for kafka > v0.8.0.
Fix up log.retention_{hours,minutes} configuration attribute
Add `log cleanup policy` configuration attribute

Only for kafka > v0.8.0.
Remove duplicated tests
Clean up capitalisation of comments in `server` template
Add helper method for conditional logic in `server` template
Add helper method for configuration attributes for `server` template

Also renamed `fetch` / `producer` attributes and squashed a bug with
`offset_metadata_max_bytes` which was defined outside the `kafka`
namespace.
Remove unnecessary chained values in tests for `_configure`
Merge branch 'configuration-differences'
Don't set `checksum` and `md5_checksum` if not already set