Have the MCO manage the master ignition stub config on baremetal

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.

While currently we don't have an automated way of recreating master
nodes, on the baremetal platform we now ship an up-to-date image as part
of the release payload, and it would be expected that any reprovisioning
of Machines would use this image.

Therefore, on the baremetal platform, 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, on
all platforms) was previously committed in
8d278d2, but later reverted by
3920ae4.

pkg/asset/machines/master.go

@@ -361,6 +361,9 @@ func (m *Master) Generate(dependencies asset.Parents) error {
 		mpool.Set(pool.Platform.BareMetal)
 		pool.Platform.BareMetal = &mpool
 
+		// Use managed user data secret, since we always have up to date images
+		// available in the cluster
+		masterUserDataSecretName = "master-user-data-managed"
 		machines, err = baremetal.Machines(clusterID.InfraID, ic, &pool, "master", masterUserDataSecretName)
 		if err != nil {
 			return errors.Wrap(err, "failed to create master machine objects")