Telemetry storage usage

[joshuachp]

The storage usage information returns the device name, on the interface the parameter is called label. We should define what this parameter should be if the device label is missing

[rgallor]

I also noticed another problem with the telemetry storage: at the moment we replace the /dev/ with an empty string to avoid an Astarte error, however if there exists a storage with a path different from /dev/ (e.g., /dev/mapper/STORAGE_NAME) the replace method will not prevent from the error to occur.