Set `config_dir` and `log4j` attrs. in attributes file

So, I decided to move them back since I realised that it’s nice to
manipulate the log4j configuration before actually including the
default recipe. To get around the issue with setting the log file for
an appender I decided to use `lazy` values (either through the use of
`lazy` or by creating a lambda/proc), which I think turned out to be
a quite nice solution, and another benefit of keeping the attributes
in the attributes file is that all defaults (or at least close to all)
are defined in the same place.
Make `jvm_performance_opts` default more readable
Install Kafka into a version specific directory

To facilitate upgrading a Kafka node it’s somewhat necessary to install
each version into each own directory and then maintain a symlink to the
currently installed version. If there had been an actual package to
install that would’ve simplified things, but alas there’s none at the
moment that’ll cover all supported platforms, so this is the next best
thing.

Furthermore the home directory of the `kafka` user has been changed to
`/var/empty/kafka` rather than the install directory. Why this particular
directory you might ask? Well, I inspected /etc/passwd on CentOS 6.5 and
noticed that some (service) users used /var/empty/<service> as their
home directory, so I thought why not?
Extract variables in `_install` recipe to helpers
Remove comment in `_coordinate` recipe