Match Swarm in how to combine filters

`docker stack services --filter=label=foo=bar --filter=label=foo=baz my-stack` with Swarm gets handled as `filter on (a label named foo with value bar) AND (a label named foo with value baz).
This obviously yields an empty result set every time, but if and how this should be changed is out of scope here, so simply align Kubernetes with Swarm for now.

Signed-off-by: Mathieu Champlon <mathieu.champlon@docker.com>

cli/command/stack/kubernetes/services.go

@@ -24,14 +24,11 @@ var supportedServicesFilters = map[string]bool{
 func generateSelector(labels map[string][]string) []string {
 	var result []string
 	for k, v := range labels {
-		switch len(v) {
-		case 0:
+		for _, val := range v {
+			result = append(result, fmt.Sprintf("%s=%s", k, val))
+		}
+		if len(v) == 0 {
 			result = append(result, k)
-		case 1:
-			result = append(result, fmt.Sprintf("%s=%s", k, v[0]))
-		default:
-			sort.Strings(v)
-			result = append(result, fmt.Sprintf("%s in (%s)", k, strings.Join(v, ",")))
 		}
 	}
 	return result