Properly create parent nodes in reachability builders

The abstract class quietly relied on parent nodes being created by the
datastore, which could have lead to them not being cleaned up. Fix the
code to do the right thing.

Change-Id: Ie5dd735b4a0791a4439d446cb2a7a5bb6fe87db5
Signed-off-by: Robert Varga <rovarga@cisco.com>