Unable to use newer JVM memory settings as Xmx overrides

The Zookeeper environment initialization scripts forcibly set an Xmx setting. This is an issue for more flexible configuration JVM memory options like (MaxRAMPercentage and InitialRAMPercentage) as Xmx overrides them.

Ideally, if SERVER_JVMFLAGS/CLIENT_JVMFLAGS are set, we should not attempt to modify them as the operator has decided what flags to set. However, this would be a behavior change, I don't know if the project is interested/willing to do that. Alternatively, we add a flag to disable the default (current) behavior.

# default heap for zookeeper server
if [ "$ZK_SKIP_XMX" != "yes" ];
  ZK_SERVER_HEAP="${ZK_SERVER_HEAP:-1000}"
  export SERVER_JVMFLAGS="-Xmx${ZK_SERVER_HEAP}m $SERVER_JVMFLAGS"
fi

Repeat for CLIENT_JVMFLAGS.

Thank you