Description of problem:

etcd-to-ephemeral procedure will be GA on 4.17 release. On QA testing, it's observed that the TP procedure in 4.16 details a step that is not actually needed:

5. Edit the ControlPlaneMachineSet (CPMS) to add the additional block ephemeral device that is used by etcd by running the following command: [...]

That should be omitted or replaced by a flavor change in CPMS object updating the control-plane nodes with a flavor that includes an ephemeral value.

Therefore, considering below flavors:

$ openstack flavor list | grep -e master -e ID
| ID                                   | Name        |   RAM | Disk | Ephemeral | VCPUs | Is Public |
| 12e1ef8d-c6d9-4499-bc61-a06dfa24a17f | master.noephemeral | 16384 |   40 |         0 |     4 | True      |
| ed39d66c-c240-4dec-b2ab-384e336f7a0d | master.ephemeral   | 16384 |   40 |        10 |     4 | True      |

I see two options:

• Deploy directly with master.ephemeral flavor in the control plane, so no change on CPMS object is needed.
• Deploy with master.noephemeral flavor and then run (where $CONTROL_PLANE_FLAVOR="master.ephemeral"):

$ oc patch ControlPlaneMachineSet/cluster -n openshift-machine-api --type json -p ' 
[
    {
      "op": "add",
      "path": "/spec/template/machines_v1beta1_machine_openshift_io/spec/providerSpec/value/flavor", 
      "value": "'${CONTROL_PLANE_FLAVOR}'"
    }
  ]
'

and in all cases, it's not needed to add the additionalBlockDevices attribute in the CPMS object and it is always needed to load the machineConfig in the system.