Check device existence

[dtaniwaki]

I found a problem that a pod w/ host device resources can get deployed to a node thanks to this device plugin but failed to use it if the node actually doesn't have the device. I made it unhealthy. We still need to dynamically change the health by device existence changes.

[everpeace]

How about returning empty device list instead of unhealthy devices??

[dtaniwaki]

I made devices Unhealthy because it might come up a while later. However, to we handle the health state properly, we still need periodical health check which is not included in this PR and the implementation is not completed. I'll make the change in this PR as well.

[everpeace]

lgtm. thanks!