Add README for network configuration

A rather self-explanatory commit, I hope, which adds a README explaining how
things in the vcloud/net folder work.
Add Edge Gateway firewall and NAT rule config file

This commit adds an Edge Gateway firewall and NAT rule configuration file. The
syntax is, hopefully, self-explanatory, however full documentation can be found
in the `vcloud-edge_gateway` repository's README on GitHub.
Add interfaces.yaml file

In order to ensure that the edge gateway can be configured correctly, the
interfaces file is required. This commit adds only one interface file for
Carrenza - the interface attached to the Edge Gateway in the BACKUP vDC.
Add YAML to create Org vDC networks

This commit adds a YAML file in order to create org vDC networks. Org vDC
networks are required in order to ensure traffic from a vDC can be routed via an
Edge Gateway.