More kafka server options

This change includes a few style changes and adds two new options to `kafka::server`:

```puppet
# $java_home                            - Value for the JAVA_HOME environment variable.  Default: undef
```

and

```puppet
# $num_partitions                       - The default number of partitions per topic.
#                                         Default: size($log_dirs)
```

I tested this change without setting either option to verify the no-op behavior, as well as the correctness of setting both options.