Handle private docker registry auth

This adds support for private docker registries, by adding
an optional `registryAuth` field in the CreateFunctionRequest.
Auth must be passed as base64-encoded basic auth, similar to
how done in Docker file store credentials (~/.docker/config.json).
Credentials are then passed to swarm at service creation.

gateway/requests/requests.go

@@ -18,6 +18,11 @@ type CreateFunctionRequest struct {
 
 	// EnvVars provides overrides for functions.
 	EnvVars map[string]string `json:"envVars"`
+
+	// RegistryAuth is the registry authentication (optional)
+	// in the same encoded format as Docker native credentials
+	// (see ~/.docker/config.json)
+	RegistryAuth string `json:"registryAuth,omitempty"`
 }
 
 type DeleteFunctionRequest struct {