Add bcache-tuning charm absence warning

[zxhdaze]

This charm [1] is being used in a lot of deployments, and recently we had a cloud which has Ceph OSD and nova-compute charms placed on the different hardware, while bcache-tuning was related only with the OSD units.

It will be nice to have this check in juju-lint, so operator could be at least aware that there is potential performance optimisation missing.

[1] https://jaas.ai/bcache-tuning