Yaml configuration system refactoring.

  * Refactoring YamlInterpolator -> YamlRefLoader
  * Add new exception types with formating errros, specified for cfg
  * Add documentation in class docstrings
  * Add unit tests for cfg.YamlRefLoader and exceptions.YamlException
  * Extend reference links possibilities (concatenating, etc.)
  * Add refence loop control
  * Reverting to simpler reflinks using I. Melnikov's draft (thanks to him).

Change-Id: I565085ae9a3015d005dcc532a74aaca49f41277a