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.

Adapt PR to nested taxonomies

Do not set default taxonomy if taxonomy already set

Removed unused users.js

Default taxonomies don't overwrite taxonomies