Add a sample network-environment.yaml file to environments

Add a sample network-environment.yaml file to environments

This change adds a sample network-environment.yaml file to the
environments. This sample includes pointers to NIC config files,
as well as default network subnets and allocation pools.

This is meant to be a demonstration of the default settings for
a virtual deployment. In a real deployment, the operator would
customize the settings here and point to custom NIC config
templates.

Change-Id: I0288c0680effea06b5f805a0d955e8bbf6152ba6