Enhancement: explicit fail when something is empty or nil

cc @fh
Fix: use doc_root_dir and not whatever is in node['vagrant']

cc @naderman
Fix: rubocop
Enhancement: unify on app_dir from EasyBib::Config

 * avoid constructing paths inside the template
 * refactor vagrant setup to avoid "current" dir hacks, etc.

cc @gusano, @naderman
Fix: style
Update: disable PerceivedComplexity for get_appdata
Enhancement: better debug message
Fix: directory has trailing slash
Fix: use get_domains()
Update: remove this from spec as we no longer need it