Add direct_functions mode to gateway for tuning

Adds a pair of configuration options for performance tuning. The
gateway can now invoke functions directly and can bypass the
provider. See updated table in README.md for configuration values.

BaseURLResolver is added with unit tests that decouples resolving
upstream URL from the reverse proxy client code.

- SingleHostBaseURLResolver resolves a single upstream host
- FunctionAsHostBaseURLResolver resolves host based upon conventions
within the URL of the request to a function for direct access

Tested with Kubernetes (faas-netes) and faas-swarm through UI, CLI
calling system endpoints and functions directly.

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

gateway/types/readconfig.go

@@ -102,6 +102,9 @@ func (ReadConfig) Read(hasEnv HasEnv) GatewayConfig {
 		cfg.PrometheusHost = prometheusHost
 	}
 
+	cfg.DirectFunctions = parseBoolValue(hasEnv.Getenv("direct_functions"))
+	cfg.DirectFunctionsSuffix = hasEnv.Getenv("direct_functions_suffix")
+
 	return cfg
 }
 
@@ -131,6 +134,12 @@ type GatewayConfig struct {
 
 	// Port to connect to Prometheus.
 	PrometheusPort int
+
+	// If set to true we will access upstream functions directly rather than through the upstream provider
+	DirectFunctions bool
+
+	// If set this will be used to resolve functions directly
+	DirectFunctionsSuffix string
 }
 
 // UseNATS Use NATSor not