Properly start ns_server_sup as part of ns_server:restart.

Change-Id: If289c95a45074aff98751ecec36956d652b09b7d
Reviewed-on: http://review.couchbase.org/36284
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>