Custom certificate authorities for S3

As an OpenShift administrator
I want to provide the registry operator with a custom certificate authority for S3 storage
so that I can use a third-party S3 storage provider.

Acceptance criteria

1. Users can specify a configmap name (from openshift-config) in config.imageregistry/cluster's spec.storage.s3.
2. The operator uses CA from this configmap to check S3 bucket.
3. The image registry pod uses CA from this configmap to access the S3 bucket.
4. When a custom CA is defined, the operator/image-registry should still trust certificate authorities that are used by Amazon S3 and other well-known CAs.
5. An end-to-end test that runs minio and checks the image registry becomes healthy with it.