MB-100: close shell connection in wait_service_started

Change-Id: Id5b3a852e01f7e0917afdc127fbb55296e262eaa
Reviewed-on: http://review.couchbase.org/24964
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>