CBQE-0: modified rest_client_tasks to user COUCHBASE_SSH_USER, COUCHBASE_SSH_PASSWORD instead as we have different ssh user and password for linux and windows vms"

Change-Id: Id1129c0aaf5d103dc62e8456aeb53c044e548fda
Reviewed-on: http://review.couchbase.org/41643
Reviewed-by: Aruna Piravi <aruna@couchbase.com>
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>