Bug 8449 - BindingToNormalizedNodeCodec fails to deserialize union of leafrefs

Fix problem of leafref in typedef called from union
 *generated part
   *generating of new property of GTO for leaf's union type of typedef
     according to return type of referenced leaf via leafref from typedef
 *codec part
   *getting codec of leaf type according to new generator part of leafref
    in typedef
 *tests

Change-Id: Ibffe4e51ef66f1911c32c71d4f08bbdbdd40e234
Signed-off-by: Jakub Toth <jakub.toth@pantheon.tech>