Make ufscluster singleton for the entire jvm process

Since it is too time consuming to finish the whole integration test over the mini-dfs cluster, here this PR aims to speed up the integration test by sharing the single ufs-cluster instance for all the test cases. The basic idea is to start the underfs-cluster once(for the first time when localtachyoncluster is up) and cleanup the test environment after each unit test (instead of turning on/off it from time to time). And finally to shutdown the underfs-cluster service while the JVM process exists. Now the integration test only takes 3x of the unit test, which is much better than before.