Implement server-side rollback, for daemon versions that support this

Server-side rollback can take advantage of the rollback-specific update
parameters, instead of being treated as a normal update that happens to
go back to a previous version of the spec.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

service_update.go

@@ -27,6 +27,10 @@ func (cli *Client) ServiceUpdate(ctx context.Context, serviceID string, version
 		query.Set("registryAuthFrom", options.RegistryAuthFrom)
 	}
 
+	if options.Rollback != "" {
+		query.Set("rollback", options.Rollback)
+	}
+
 	query.Set("version", strconv.FormatUint(version.Index, 10))
 
 	var response types.ServiceUpdateResponse