We now require PuppetDB 1.5 / API v3.

PUPPETDB_API is no longer configurable since we're now using features
that are v3 only. Limiting ourselves to v2 compatibility is far too
troublesome and people tend to update to newer versions of PuppetDB
fairly quickly.
screenshots: New overview and nodes pages.
Get rid of old settings.

I broke things with 795d243e9d998d917282371cbf5bac7c9f0e6d83 because I
forgot to remove it everywhere from functions and templates. Also
removed the old PUPPETDB_EXPERIMENTAL switching.