Add provider network link from a virtual network
Add provider network link from a virtual network

The link determines which routing instance is used to forward the traffic
of the virtual-network. To begin with, only IP fabric network can be chosen
as the provider network of any VN.

Change-Id: Ib4035db429c7b58f0b1f0e92e6feb8d903c6c459
partial-bug: #1702837
Implements: blueprint gateway-less IP fabric forwarding

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