Have the MCO manage the master ignition stub config

Since openshift/machine-config-operator#2827, the MCO creates managed
master and worker ignition stub configs to ensure they always use the
latest version of the ignition format.

Create new master Machines using the master-user-data-managed Secret,
and install the initial user data in this Secret instead of the old
master-user-data Secret.

This is the part of the installer changes for the enhancement:
https://github.com/openshift/enhancements/blob/master/enhancements/machine-config/user-data-secret-managed.md

A previous version of this patch (but for both masters and workers) was
previously committed as 8d278d2, but
later reverted by 3920ae4.

pkg/asset/machines/master.go

@@ -157,7 +157,7 @@ func (m *Master) Generate(dependencies asset.Parents) error {
 	mign := &machine.Master{}
 	dependencies.Get(clusterID, installConfig, rhcosImage, mign)
 
-	masterUserDataSecretName := "master-user-data"
+	masterUserDataSecretName := "master-user-data-managed"
 
 	ic := installConfig.Config