hadoop_cluster: fix java heap flags for m1.large

The current memory settings are more than two times too aggressive: 6
* 2400 = 14400, but that's about twice the available memory on an
m1.large. I'm seeing that there is something like 5.8G free on an
m1.large after spinning up a datanode and a tasktracker. I'm setting
these flags conservatively to account for additional processes that
may be spun up. If this number is too large, the kernel will forcibly
kill the process responsible for handling the Hadoop task.