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.

However, since the storage role is applied twice, including just before
the ring-compute role, if the ring-compute role fails to apply, then we
will hit this error and we will not be able to get the ring-compute role
re-applied. And therefore we will be stuck here.

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

Part of https://bugzilla.novell.com/show_bug.cgi?id=861089
Merge pull request #195 from vuntz/missing-ring

Do not fail on storage nodes if rsync of rings fails