Starting replicator/reconstructor with *_workers and no devices in ring (yet) runs inline

[clayg]

I've had reports that starting a replicator/re-constructor after "adding a new node" ignores the workers option.

The code looks for locals devices to divvy them up between the available workers. But if there's no devices it returns an empty set and falls back to inline mode [1].

Once the deamon goes into inline mode, if new rings come down with devices it just restarts the run loop - it doesn't bounce out to the daemon strategy to redeploy workers.

1. https://github.com/openstack/swift/blob/b4c8cf192a39004645e2608d4cfc0ac5dcf3b516/swift/common/daemon.py#L224