Extract scaling from zero

- extracting this package means it can be used in other components
such as the asynchronous nats-queue-worker which may need to
invoke functions which are scaled down to zero replicas.

Ref: https://github.com/openfaas/nats-queue-worker/issues/32

Tested on Docker Swarm for scaling up, already scaled and not
found error.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

gateway/scaling/service_query.go

@@ -0,0 +1,19 @@
+// Copyright (c) OpenFaaS Author(s). All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+package scaling
+
+// ServiceQuery provides interface for replica querying/setting
+type ServiceQuery interface {
+	GetReplicas(service string) (response ServiceQueryResponse, err error)
+	SetReplicas(service string, count uint64) error
+}
+
+// ServiceQueryResponse response from querying a function status
+type ServiceQueryResponse struct {
+	Replicas          uint64
+	MaxReplicas       uint64
+	MinReplicas       uint64
+	ScalingFactor     uint64
+	AvailableReplicas uint64
+}