CBQE-0: close RemoteMachineShellConnection in installer

Change-Id: I996e3d7371c42e50bb8604ad49b426e19f4aa01a
Reviewed-on: http://review.couchbase.org/26257
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>