Fix help message flags on docker stack commands and sub-commands

PersistentPreRunE needs to be called within the help function to initialize all the flags (notably the orchestrator flag)
Add an e2e test as regression test

Signed-off-by: Silvin Lubecki <silvin.lubecki@docker.com>

e2e/stack/help_test.go

@@ -0,0 +1,24 @@
+package stack
+
+import (
+	"fmt"
+	"testing"
+
+	"gotest.tools/golden"
+	"gotest.tools/icmd"
+)
+
+func TestStackDeployHelp(t *testing.T) {
+	t.Run("Swarm", func(t *testing.T) {
+		testStackDeployHelp(t, "swarm")
+	})
+	t.Run("Kubernetes", func(t *testing.T) {
+		testStackDeployHelp(t, "kubernetes")
+	})
+}
+
+func testStackDeployHelp(t *testing.T, orchestrator string) {
+	result := icmd.RunCommand("docker", "stack", "deploy", "--orchestrator", orchestrator, "--help")
+	result.Assert(t, icmd.Success)
+	golden.Assert(t, result.Stdout(), fmt.Sprintf("stack-deploy-help-%s.golden", orchestrator))
+}