Enable backoff/retries on scaling up

- this change is needed for Docker Swarm which may give an error
when several concurrent requests come in to scale a deployment.

Tested on Docker Swarm before/after with the hey tool and figlet
scaled down to zero replicas.

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

gateway/scaling/scaling_config.go

@@ -9,7 +9,8 @@ type ScalingConfig struct {
 	// MaxPollCount attempts to query a function before giving up
 	MaxPollCount uint
 
-	// FunctionPollInterval delay or interval between polling a function's readiness status
+	// FunctionPollInterval delay or interval between polling a function's
+	// readiness status
 	FunctionPollInterval time.Duration
 
 	// CacheExpiry life-time for a cache entry before considering invalid
@@ -17,4 +18,8 @@ type ScalingConfig struct {
 
 	// ServiceQuery queries available/ready replicas for function
 	ServiceQuery ServiceQuery
+
+	// SetScaleRetries is the number of times to try scaling a function before
+	// giving up due to errors
+	SetScaleRetries uint
 }