BUG 2468 - IdentityValuesDTO to string class cast exception.

If leaf had type which is definined in other module (imported) via typedef
and this type is leafref then it wasn't possible correctly resolve base
type of referenced type.

New method for this resolution was implemented and is now called.

Also test for this case was added

Merge precondition:
yangtools: https://git.opendaylight.org/gerrit/13683

Change-Id: If00db3a6297b5a0ea657acb469f4f49f350f8c63
Signed-off-by: Jozef Gloncak <jgloncak@cisco.com>