JobTracker must ask for a new FS instance and close it when terminated.

This is something I've been experimenting with HADOOP-3268; I'm not sure what the right action is here.

-currently, the JobTracker does not close() its filesystem when it is shut down. This will cause it to leak filesystem references if JobTrackers are started and stopped in the same process.

-The TestMRServerPorts test explicitly closes the filesystem
jt.fs.close();
jt.stopTracker();

-If you move the close() operation into the stopTracker()/terminate logic, the filesystem gets cleaned up, but
TestRackAwareTaskPlacement and TestMultipleLevelCaching fail with a FilesystemClosed error (stack traces to follow)

Should the JobTracker close its filesystem whenever it is terminated? If so, there are some tests that need to be reworked slightly to not expect the fileystem to be live after the jobtracker is taken down.