Add check for namespace label openfaas=true

This commit adds the checks that the namespace supplied by the user has
the `openfaas=true` label. Without this check the user can
deploy/update/read functions in any namespace  using the CLI.

The UI is not effected because it calls the listnamesaces endpoint,
which has the check for the label

Signed-off-by: Alistair Hey <alistair@heyal.co.uk>

pkg/provider/handlers/functions.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log"
 	"strings"
@@ -32,6 +33,17 @@ type Function struct {
 
 // ListFunctions returns a map of all functions with running tasks on namespace
 func ListFunctions(client *containerd.Client, namespace string) (map[string]*Function, error) {
+
+	// Check if namespace exists, and it has the openfaas label
+	nsValid, err := validateNamespace(client, namespace)
+	if err != nil {
+		return nil, err
+	}
+
+	if !nsValid {
+		return nil, errors.New("namespace not valid")
+	}
+
 	ctx := namespaces.WithNamespace(context.Background(), namespace)
 	functions := make(map[string]*Function)