Update DBSerializer for relationships retrieving

Update DBSerializer for relationships retrieving

Enhance the code for how the db serializer works
when it comes to dealing with the relationships
For a particular vertex, when retrieving its relationship vertexes
it doesn't need to get all the properties of the relationship vertex
as it only cares about the aai-uri and named properties
so only partially retrieving those props that is needed for relationships

Issue-ID: AAI-1987
Change-Id: I1440c561d187ff180a599a3f52ac5c8715d5c8e3
Signed-off-by: Harish Venkata Kajur <vk250x@att.com>