nodetool: ignore calculated JVM heap size
nodetool: ignore calculated JVM heap size

Ignore the calculations in cassandra-env.sh (which are meant for Cassandra,
not nodetool) and use a reasonable size. Use serial GC too, which is more
efficient.
Message-Id: <9ebe6726cb3b856eaab33d1cfc1faca4e23b1181@scylladb.com>

scylladb/scylla-tools-java,scylladb/scylla-tools-java,scylladb/scylla-tools-java,scylladb/scylla-tools-java