Make ns_server:restart/0 ensure that ns_server_sup starts.

Change-Id: I15522279c50072cd28dbfbb23f8f8439eb5fe999
Reviewed-on: http://review.couchbase.org/36161
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>