Expand insserv variables.

def _InsservExpander(self, facilities, val):
    """Expand insserv variables."""
    expanded = []
    if val.startswith("$"):
      vals = facilities.get(val, [])
      for v in vals:
        expanded.extend(self._InsservExpander(facilities, v))
    elif val.startswith("+"):
      expanded.append(val[1:])
    else:
      expanded.append(val)
    return expanded