nodetool: reduce memory usage
nodetool: reduce memory usage

Nodetool calls `java -version` to ensure it can run. However, that requires about
1GB of RAM to calculate, which may not be available if swap is disabled, as Scylla
allocates most of memory.

Fix by limiting the heap size of the `java -version` call, reducing memory usage
to a tiny 130MB.

Fixes #52
Message-Id: <7f3ae25dc425f7ab90cc21a7c7f70ea298c38178@scylladb.com>

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