ibmcloud: revise ibmcloud README

[genjuro214]

I followed the guide to Deploy the Confidential-containers operator on a self-managed cluster in IBM Cloud VPC. However, it doesn't work as expected. I try to fix 2 problems:

• Fix the default VPC service url, revert it to the value in v0.8.0: https://github.com/confidential-containers/cloud-api-adaptor/blob/v0.8.0/pkg/adaptor/cloud/ibmcloud/provider.go#L79

• Add missing settings in selfmanaged_cluster.properties. Without those settings, CAA will fail to create instance with errors like below:

failed to create an instance : Expected only one oneOf fields to be set: got 0 and the response is {

"code": "validation_failed_oneof",
                "message": "Expected only one oneOf fields to be set: got 0",
                "target": {
                    "name": "InstancePrototype",
                    "type": "field"
                }

"message": "InstanceByImage is invalid: Expected only one oneOf fields to be set: got 0",

[stevenhorsman]

The changes look fine. Just to note that these won't fix the issues I'm seeing with #1882 as I've been providing the extra properties in self_managed.properties and the VpcServiceUrls is overwritten in our e2e test code to the correct value.

[liudalibj]

LGTM