Add health and info endpoints

Fixes issue 689 by enabling /healthz and /system/info, see
swagger for more details.

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

api-docs/swagger.yml

@@ -1,7 +1,7 @@
 swagger: '2.0'
 info:
   description: OpenFaaS API documentation
-  version: 0.8.0
+  version: 0.8.1
   title: OpenFaaS API Gateway
   license:
     name: MIT
@@ -213,6 +213,29 @@ paths:
           description: Function not found
         '500':
           description: Error querying function
+  '/system/info':
+    get:
+      summary: Get info such as provider version number and provider orchestrator
+      produces:
+        - application/json
+      responses:
+        '200':
+          description: Info result
+          examples:
+            application/json: |-
+                {"provider":"faas-swarm","version":{"sha":"7108418d9dd6b329ddff40e7393b3166f8160a88","release":"0.2.6"},"orchestration":"swarm"}
+        '404':
+          description: Provider does not support info endpoint
+        '500':
+          description: Error finding info
+  '/healthz':
+    get:
+      summary: Healthcheck
+      responses:
+        '200':
+          description: Healthy
+        '500':
+          description: Not healthy
 definitions:
   DeleteFunctionRequest:
     type: object

gateway/server.go

@@ -28,7 +28,7 @@ func main() {
 	log.Printf("HTTP Write Timeout: %s", config.WriteTimeout)
 
 	if !config.UseExternalProvider() {
-		log.Fatalln("As of this version of OpenFaaS, you must use external provider even for Docker Swarm.")
+		log.Fatalln("You must provide an external provider via 'functions_provider_url' env-var.")
 	}
 
 	log.Printf("Binding to external function provider: %s", config.FunctionsProviderURL)
@@ -92,6 +92,8 @@ func main() {
 	r.HandleFunc("/function/{name:[-a-zA-Z_0-9]+}", faasHandlers.Proxy)
 	r.HandleFunc("/function/{name:[-a-zA-Z_0-9]+}/", faasHandlers.Proxy)
 
+	r.HandleFunc("/system/info", handlers.MakeForwardingProxyHandler(reverseProxy, forwardingNotifiers, urlResolver)).Methods(http.MethodGet)
+
 	r.HandleFunc("/system/alert", faasHandlers.Alert)
 
 	r.HandleFunc("/system/function/{name:[-a-zA-Z_0-9]+}", queryFunction).Methods(http.MethodGet)
@@ -117,6 +119,8 @@ func main() {
 
 	metricsHandler := metrics.PrometheusHandler()
 	r.Handle("/metrics", metricsHandler)
+	r.HandleFunc("/healthz", handlers.MakeForwardingProxyHandler(reverseProxy, forwardingNotifiers, urlResolver)).Methods(http.MethodGet)
+
 	r.Handle("/", http.RedirectHandler("/ui/", http.StatusMovedPermanently)).Methods(http.MethodGet)
 
 	tcpPort := 8080