Moved 'system' inputs out as individual inputs

[sdomino]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

This is in response to a comment made in #4290. Associated docs update here.

I wasn't sure whether to separate net and netstat. After looking them over, it seemed like they're intended to fulfill the same purpose for different platforms, and so would remain as parts of the same package.

Ultimately, I based that decision off of reading the following in the net README:

Different platforms gather the data above with different mechanisms. Telegraf uses the (gopsutil) package, which under Linux reads the /proc/net/dev file. Under freebsd/openbsd and darwin the plugin uses netstat.

[danielnelson]

Can you leave this file in the system package for the time being? Long term I would like to split it out to the required plugins as well as splitting the interface up, but we should do that after moving the files to reduce the size of the conflict window.

[danielnelson]

Leave in system package for now.

[tstackhouse]

It was mentioned in #4400 that the system package may be removed soon, with this PR, and with 1.8 on the horizon, what would be the equivalent? Assuming the docs will be updated soon?

[danielnelson]

This is mostly a code organization change. From a users point of view the only change is that the README files for the plugins have been moved to a different location. For example, with the disk input:

- /plugins/inputs/system/DISK_README.md
+ /plugins/inputs/disk/README.md

There isn't any effect on the configuration file or on plugin availability, and the system input plugin is not removed, so there should be no action required.