Core:
   - Support singletons (see types/issue for a good reason)
      - probably name them only_once and use that as the internal id!
   - extend cdist_tree_wrapper to look for optional/mandority parameters for types
      - gives "good" database, which contains only valid objects
   - use ssh with master socket?
      - saves connection delay for new connections: -M
      - hint: http://protempore.net/~calvins/howto/ssh-connection-sharing/
   - remove remote_base_dir at beginning - otherwise fragments from
     previous runs may still be existing!

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?)