Support PuppetDB API v3.

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.