Align sm-agent and Scylla object storage configurations

[Michal-Leszczynski]

The description of this issue and possible solutions can be found in this doc.

The problem is that sm-agent configures object storage access via scylla-manager-agent.yaml, but Scylla uses object_storage.yaml for this purpose. Those files have different formats and it's not obvious how to match object storage configurations between them.

One of the possible solutions to this problem is #4161, but we haven't yet decided if that's the way to go.