Bug 1991: Ignore AugmentationIdentifier in XML Serialization

Bug 1991: Ignore AugmentationIdentifier in XML Serialization

AugmentationIdentifier is YANGTools/MD-SAL specific
concept which eases navigation in parsed data, but
does not exists in XML Instance Identifier.

AugmentationIdentifier from XML perspective identifies
subset of child nodes of parent element, from which
also next path argument select element from that subset
so it is safe to leave it during serialization.

Change-Id: If42acf6db82f7c4920546283cb89a2cf58f80a6e
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
(cherry picked from commit a24c4089733f94528a3e2f4af9bdc6102974bf53)