Support for cassandra v2 and vnodes

- adds USE_VNODES config for v1.2 and v2
- adds numTokensPerNode config
- adds checkForAndInstllJava7or8
- make AstyanaxSupport more robust for testing
Adds JmxHelper.terminate()

- important for entity stop(), if a connect attempt is taking a very
  long time to timeout and there are other jobs backing up.