Allow for -XX:MaxRAMPercentage JVM setting

When running Zk in a containerized environment, it's sometimes desirable to express your heap size in terms of percentage of available memory allocated to a container.

As it stands, zkEnv.sh forces your to have  -Xmx, which defaults to 1GB. Some environments wanted to set it to more, mostly related to the amount of Ram.

This is a request to implement the option of using -XX:MaxRamPercentage option when starting zookeeper.

Suggested implementation is to also make a variable ZK_SERVER_MAXRAMPERCENTAGE available to be added to SERVER_JVMFLAGS. If the variable is set, ZK_HEAP_SERVER is ignored, if no ZK_SERVER_MAXRAMPERCENTAGE, ZK_SERVER_HEAP is set as usual.