Bug #3214 No taxonomy if host added from puppet rn

When a host is created through a puppet run, no taxonomies are assigned to it.
This PR solves the issue of taxonomy-less hosts in 2 different ways.
1. Default taxonomy
  From now on Setting[:default_taxonomy] will be the taxonomy of choice
  for hosts that are created through a puppet run.
2. Taxonomy from facts
  Two facts, defined by Setting[:taxonomy_fact], will be available to
  users so that they can define these facts in their own hosts. If
  there's a taxonomy in Foreman with the same name as the fact value
  the host will be placed in that taxonomy.

Set taxonomy from facts

Update descriptions of taxonomy facts