reuse cluster for TFsShellTest and made each test isolated

Cut TFsShellTest from 6 minutes to 30 seconds.  Same trick can be used for all long running tests.
Class and member modifiers, when present, appear in the order recommended by the Java Language Specification: public protected private abstract static final transient volatile synchronized native strictfp, then as alphabetical order.

http://tachyon-project.org/master/Startup-Tasks-for-New-Contributors.html