CBQE-0: RemoteMachineShellConnection methods shouldn't close connection

fix base on review http://review.couchbase.org/#/c/36800/3/lib/remote/remote_util.py

Change-Id: I5835144459f307545f1368e284c678d6d058dc46
Reviewed-on: http://review.couchbase.org/36827
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>