zookeeper should set a default value for SERVER_JVMFLAGS and CLIENT_JVMFLAGS so that memory usage is controlled

We noticed this with jdk 1.6 where if no heap size is set the process takes up to 1/4 of mem available on the machine.

More info http://stackoverflow.com/questions/3428251/is-there-a-default-xmx-setting-for-java-1-5

You can run the following command to see what are the defaults for your machine

java -XX:+PrintFlagsFinal -version 2>&1 | grep -i -E 'heapsize|permsize|version'

And we noticed on two different class of machines that this was 1/4th of total memory on the machine.