Add device name as a tag in disk stats

[mediocretes]

Required for all PRs:

• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• Sign CLA (if not already signed)
• README.md updated (if adding a new plugin)

This returns device name as a tag (alongside mount point), and removes /dev/ from the device name for ease of reading.

[phemmer]

You might want to look at #1453. Currently it only affects diskio inputs. But it should be easy to include it on the disk inputs as well.

[mediocretes]

@sparrc Any guidance?

If this isn't going to make it, we need to think about how our backend is going to handle mount points without sending some really odd data.

[sparrc]

thanks for reminding, we'll try to get it ready for 1.2