Core:
   - Support singletons (see types/issue for a good reason)
   - extend cdist_tree_wrapper to look for optional/mandority parameters for types
      - gives "good" database, which contains only valid objects

Type handler:
   - add dependency parameters to core available for every type
      --requires
      --excludes?

Stage 5 (code execution):
   - check return codes
   - abort on first error?
   - dependencies


Types to be written:
   - ssh-keys (host/user)
   - services
   - user management
   - file editing 
      - add_line_to_file_if_not_existing
      - delete_line_from_file
      - regexp replace (can probably cover all?)