Bug-1180: Segment Routing parsers/serializers

-modified segment-routing yang model:
	-message's ero subobject type is augmented with sr-ero subobject
-added parser/serializer for sr-ero subobject + junit tests
-added object, subobject parsers/serializers to the SegmentRoutingActivator
-fixed extended Srp and RP object serializers to handle all PST Tlv augmentations

Change-Id: Ie0241331572b5cd6a1ee6e4cd9ca1778ee445baa
Signed-off-by: Milos Fabian <milfabia@cisco.com>
Merge "Bug-1180: Segment Routing parsers/serializers"