Improve support for FOSS puppet dashboards

[suckatrash]

This addresses #47. Before this commit, the Telegraf Puppetserver Performance dashboard had pe- prefixed metrics, and didn't work with FOSS puppet.

This commit adds a conditional to the puppet_metrics_dashboard::dashboards::telegraf class that points to a new dashboard without the prefixes. This is dependent on the presence of a pe_server_version fact that will not be in a FOSS install.

[Sharpie]

Pulling this into a FOSS install does result in a usable dashboard. However, I'm hemming and hawing a bit as dropping the pe- prefix does prevent the boards from working on 2017.2.x or older, along with archival data.

It seems like this problem could be solved with a constant variable that allowed pe- to be selectively set on import. However, the Grafana HTTP API appears to have no support for setting variables on import :(

The other solution would be to switch from files to templates, but that makes updating the boards more difficult going forward.

So, I suppose the question is:

• Is templating worth it?

• Should we cut a major release dropping support for Puppet 4 and PE versions older than 2017.3?

• Or, since the FOSS impact is really just Telegraf_Puppetserver_Performance.json, should we create a copy of just that template without the pe- prefix and use it if $facts['pe_server_version'] is undef?

After typing all this out, I'm starting to lean towards Option 3.

[Sharpie]

Looks good to me! Tested against Puppet Server 5 and everything is showing up 👍