redirect all SG logs in /root/gateway.log

Change-Id: I29baa92bb64743791025eda6ec9f59ea8df0e716
Reviewed-on: http://review.couchbase.org/43603
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>