Collection interval random jittering

[sparrc]

closes #460

[sparrc]

cc @sebito91 @daviesalex

[sparrc]

One thing is different from #460, which is that the jitter is simply a random sleep after interval seconds, meaning that

interval = "10s"
collection_jitter = "1s"

would have the plugin run every 10 to 11 seconds (not 9 to 11 seconds as stated in the feature request). You can obviously get 9 to 11 seconds by doing:

interval = "9s"
collection_jitter = "2s"

[sebito91]

LGTM, let's push it and try it out.

[wrigtim]

Thanks Cameron :)