kubeadm: Default to using token discovery.

Recent changes to support multiple methods for discovery meant that
"kubeadm init" no longer was sufficient and users would need to add
"--discovery token://" to achieve the same results.

Instead lets assume discovery if the user does not specify anything else
to maintain parity and the brevity of our original instructions.

cmd/kubeadm/app/apis/kubeadm/v1alpha1/defaults.go

@@ -52,4 +52,8 @@ func SetDefaults_MasterConfiguration(obj *MasterConfiguration) {
 	if obj.Networking.DNSDomain == "" {
 		obj.Networking.DNSDomain = DefaultServiceDNSDomain
 	}
+
+	if obj.Discovery.Token == nil && obj.Discovery.File == nil && obj.Discovery.HTTPS == nil {
+		obj.Discovery.Token = &TokenDiscovery{}
+	}
 }

pkg/api/testing/fuzzer.go

@@ -550,6 +550,7 @@ func FuzzerFor(t *testing.T, version schema.GroupVersion, src rand.Source) *fuzz
 			obj.API.Port = 20
 			obj.Networking.ServiceSubnet = "foo"
 			obj.Networking.DNSDomain = "foo"
+			obj.Discovery.Token = &kubeadm.TokenDiscovery{}
 		},
 		func(s *policy.PodDisruptionBudgetStatus, c fuzz.Continue) {
 			c.FuzzNoCustom(s) // fuzz self without calling this function again