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/range.go

@@ -0,0 +1,21 @@
+package scaling
+
+const (
+	// DefaultMinReplicas is the minimal amount of replicas for a service.
+	DefaultMinReplicas = 1
+
+	// DefaultMaxReplicas is the amount of replicas a service will auto-scale up to.
+	DefaultMaxReplicas = 20
+
+	// DefaultScalingFactor is the defining proportion for the scaling increments.
+	DefaultScalingFactor = 20
+
+	// MinScaleLabel label indicating min scale for a function
+	MinScaleLabel = "com.openfaas.scale.min"
+
+	// MaxScaleLabel label indicating max scale for a function
+	MaxScaleLabel = "com.openfaas.scale.max"
+
+	// ScalingFactorLabel label indicates the scaling factor for a function
+	ScalingFactorLabel = "com.openfaas.scale.factor"
+)