Make NeutronDbObjectDuplicateEntry exception more verbose

Make NeutronDbObjectDuplicateEntry exception more verbose

NeutronObjectDuplicateEntry is an exception derived from Conflict,
which is mapped to HTTPConflict. When such exception is thrown
during an API layer call, we will provide more detail to the caller
about what was exactly duplicated, and for which fields,
the information is extracted from the DB exception.

NeutronObjectDuplicateEntry is renamed into NeutronDbObjectDuplicate
to make clear it is for handling db duplicate exceptions, in
the future we could generalize to a base NeutronObjectDuplicate class
if we need separate handling for other object backings (mem, keystore,
etc).

Change-Id: I6ec5883b3456ebc842f3f7bffd8cc924293f5766