Update README to include how to import pre-created OVA from IBM COS buckets

[clnperez]

Since Red Hat now pushes OVA images for PowerVS to regional COS buckets, we should update the doc to include how to use them. This would eliminate the need for users to manually create OVA images for OCP 4.11 and later releases.

[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.