Do not try to run the proxy bits too early

There are two things:

 - if we're a storage node and we have no devices yet, then we know
   we're on the first pass of the crowbar orchestration for storage
   nodes, so we can just ignore the proxy bits

 - if we do not have the rings yet, then it means the ring-compute node
   didn't push them yet. Since we know that swift-proxy will fail
   without the rings, there's no point in doing anything. For setting
   up swift-proxy, we instead rely on the chef run after the one for
   ring-compute that crowbar will orchestrate.