Mini cluster test flakiness

Mini clusters used during HDFS unit tests are reporting test failures that do not appear to be directly related to submitted changes.  The failures are the result of either interactions between tests run in parallel, or tests which share common disk space for tests.  In all cases, the tests can be run individually serially without any errors.  Addressing this issue will simplify future submissions by eliminating the confusion introduced by these unrelated test failures.

We can apply lessons recently from TestRollingUpgrade, which was recently patched to unblock a recent submission.  The fixes involved changing the HDFS configuration to use temporary disk space for each individual tests, and using try-with-resources to ensure that clusters were shutdown cleanly.