add a finally block to close stderr/stdout

Reference issue: http://tracker.ceph.com/issues/7594
And: http://tracker.ceph.com/issues/9122

this will silence all the horrible Python thread nonsense that occurs after ceph-deploy has closed the remote connection and has nothing else to do.

Users get very confused about this because they think it has to do with something that is breaking in ceph-deploy
You don't have to pass the exception into format exc, it's arg is an optional stack depth limit (https://docs.python.org/2/library/traceback.html#traceback.format_exc)