fix(outputs.groundwork): Set NextCheckTime to LastCheckTime

[Hipska]

To avoid GroundWork to invent a value like LastCheckTime + 5 minutes

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• Updated associated README.md.
• Wrote appropriate unit tests.
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[telegraf-tiger]

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s390x.deb		linux_i386.tar.gz
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[powersj]

@Hipska have you been able to test this fix? I'm only asking since the only reference to 5 minutes was from what appears to be an example config and not used.

[Hipska]

It is something GroundWork does internally, so you won't see that in the used lib.

Preferred scenario would be the output plugin be able to know what the interval for the metrics's input plugin is, but since that is not available it seems better to have both timestamps the same.