Add check for bcaches with no caching device

[Drew Freiberger]

We should expand hw-health and/or bcache-tuning to monitor for less-than-ideal states for bcache devices such as:

/sys/block/bcache*/bcache/state contains "no cache" (critical)

/sys/block/bcache*/bcache/mode is not set as "[writeback]" (or whatever bcache-tuning charm is configured to set, if it is deployed.)

Any /sys/block/bcache*/slaves directory that contains:
  - only one disk path (only a backing store)...should match up with "no cache" state.
  - i/o erroring/missing disk paths (not sure how to check this w/out hanging monitor in d-state)

/dev/bcache* devices existing but no bcache-tuning charm deployed to the metal.