godeps: update vmware/govmomi to v0.20.1

Includes fix for SAML token auth with vSphere and zones API

Issue #77360

See also: #75742

vendor/github.com/vmware/govmomi/sts/signer.go

@@ -265,8 +265,11 @@ func (s *Signer) SignRequest(req *http.Request) error {
 		}
 		bhash := sha256.New().Sum(body)
 
-		// Port in the signature must be that of the reverse proxy port, vCenter's default is port 80
-		port := "80" // TODO: get from lookup service
+		port := req.URL.Port()
+		if port == "" {
+			port = "80" // Default port for the "Host" header on the server side
+		}
+
 		var buf bytes.Buffer
 		msg := []string{
 			nonce,