more review updates

- use /secrets for swarm secret create route
- do not specify omitempty for secret and secret reference
- simplify lookup for secret ids
- do not use pointer for secret grpc conversion

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>

client/secret_create_test.go

@@ -25,7 +25,7 @@ func TestSecretCreateError(t *testing.T) {
 }
 
 func TestSecretCreate(t *testing.T) {
-	expectedURL := "/secrets/create"
+	expectedURL := "/secrets"
 	client := &Client{
 		client: newMockClient(func(req *http.Request) (*http.Response, error) {
 			if !strings.HasPrefix(req.URL.Path, expectedURL) {