Add a JAVA_HOME to the packaged /etc/default/logstash

This seems to work around and resolve strange encoding errors of the following nature:

    #<Encoding::ConverterNotFoundError: code converter not found (ASCII-8BIT to UTF-8)>

Despite there being no charset specified anywhere in the LogStash config. A similar patch may be needed to the /etc/sysconfig files for RedHat, but as I cannot test RH packages here I'm hesitant to mess with those.