Allow non-VPNs to have underlying networks.

Certain network types, like the VCN, have underlying
networks for the purpose of data usage, but do not want to
propagate the underlying network capabilities.

Allow these networks to set underlying networks, but continue
not to propagate the capabilities.

Bug: 190620024
Test: new unit test
Change-Id: I53d6080f48707ff3c37fbfbef534284ba77a7432
4 files changed