Support JAVA_STACK variable

[jhmartin]

Logstash 1.5.4 tarball installation installs .//vendor/jruby/bin/jruby.sh / jruby.bash which includes:

if [ -z "$JAVA_STACK" ] ; then
  JAVA_STACK=-Xss2048k
fi

Other than a wholesale override of the java command line arguments there is no way to inject this environmental variable into the arguments. There should be a way to specify this size.