Recently changes were made to pypuppetdb to support, at a basic level,
the v3 API introduced with PuppetDB 1.5.
This commit changes some internals of Puppetboard to handle the new
situation:
* The experimental endpoints are gone, so is the ExperimentalDisabled
Error;
* v2 API can no longer access Reports/Events so we now check that we're
talking at least v3 API;
* Introduce a configuration setting PUPPETDB_API which takes an integer
repersenting the API version we want to talk.
