logstash::params: use /usr as JAVA_HOME on redhat

This uses the java executable in /usr/bin/java which is always present
whichever version of jdk / jre happens to be installed. Maybe it would
apply to Debian as well.
Merge pull request #2 from ckaenzig/fix-redhat-javahome

logstash::params: use /usr as JAVA_HOME on redhat