Update README to include how to import pre-created OVA from IBM COS buckets about ocp4-upi-powervs

[clnperez]

Fixed by #457

[yussufsh]

I am confused. This issue concerns public COS buckets and the pull request is about downloading qcow from a restricted RH website.

When looking at image import from public buckets we already have this feature merged as part of #412. Users can directly use these variables to import the images via automation. If you think the document providing the values for below variables [1] or the description is not well defined then we can improve on it.

[1] - https://github.com/ocp-power-automation/ocp4-upi-powervs/blob/master/docs/var.tfvars-doc.md#openshift-cluster-details

rhcos_import_image              = true                                                   # true/false (default=false)
rhcos_import_image_filename     = "rhcos-411-85-202203181612-0-ppc64le-powervs.ova.gz"   # RHCOS boot image file name available in cloud object storage
rhcos_import_image_storage_type = "tier1"                                                # tier1/tier3 (default=tier1) Storage type in PowerVS where image needs to be uploaded

I am going to close the pull request as the user doesn't need to "import pre-created OVA from IBM COS buckets" himself, automation does it for him referring to the closest cos bucket region.