Add documentation for migrating an existing cluster to CCM

[HappyTetrahedron]

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[HappyTetrahedron]

By the way, the CCM reports this on startup:

W0930 11:40:15.004570       1 main.go:71] detected a cluster without a ClusterID.  A ClusterID will be required in the future.  Please tag your cluster to avoid any future issues

I assume we'll burn that bridge when we get there.

[simu]

By the way, the CCM reports this on startup:

W0930 11:40:15.004570       1 main.go:71] detected a cluster without a ClusterID.  A ClusterID will be required in the future.  Please tag your cluster to avoid any future issues

I assume we'll burn that bridge when we get there.

cloudscale CCM reports the same, IIRC that's part of the upstream generic cloud-controller-manager implementation:

$ kubectl -n syn-cloudscale-cloud-controller-manager logs cloudscale-cloud-controller-manager-6v4jz | grep ClusterID
W0929 12:55:03.338278       1 main.go:59] Detected a cluster without a ClusterID. A ClusterID will be required in the future. Please tag your cluster to avoid any future issues

I don't think it's urgent to fix right now, but might be good to have a ticket in the backlog so we don't completely forget.

[HappyTetrahedron]

Closed in favor of appuio/openshift4-docs#428