set JAVA_HEAP_OPTS for samza container based on config memory

Previously, run-class.sh was just hardcoding it to `-Xmx768M` but now run-class.sh can use a `JAVA_HEAP_OPTS` env var. For the Samza container, we just use the config value `mesos.executor.memory.mb` which defaults to 1028. This ensures that the memory allocated by Mesos for the Samza container task and the JVM heap settings are aligned.