Do not fail on storage nodes if rsync of rings fails

This can mean that the ring-compute node is not accessible, that its
rsync service is not enabled or that the ring files are missing.

These errors can only happen if there's an error while applying the
ring-compute role on the ring-compute node, which means that the
proposal will fail to apply anyhow.

For the same reason, we now only start the services if the relevant ring
file is present.