Revert "Moving cgroup path name to field from tag to reduce cardinali…

[ghost]

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)

…ty (#1457)"

This was introducing a regression with influxdb output, leading to
collision and points missing.
This reverts commit 53f4006.

Fixes #1724

[sebito91]

If you keep in mind that high cardinality can literally cause your influxdb to topple, and you're able to limit the unique pathnames you collect with then I'm sure this PR should be fine.

[sparrc]

@ririsoft for me I think it's OK to merge this because I think the cgroup plugin is substantially less useful without it.

That being said, can you add some documentation in SampleConfig and the README on limiting the cardinality that this plugin creates?

@sebito91 @daviesalex do you have any input on what patterns to avoid in the configuration of this plugin? This is the current README, and I'm not sure if the documented patterns along with using the paths as tags should be considered dangerous: https://github.com/influxdata/telegraf/tree/master/plugins/inputs/cgroup

[ghost]

@sparrc documentation and changelog updated as suggested. I took the opportunity to rebase against master. Cheers.