Use `install_dir` as home dir. for Kafka user

Add a test so that `/home/kafka` isn’t created.
Build Kafka in temporary directory
Move `gc_log_opts` attribute to `_defaults` recipe
Move `config_dir` to `_defaults` recipe
Move log4j config. attributes to `_defaults` recipe
Move inclusion of `_defaults` to `default` recipe
Remove comments+whitespace from `_defaults`