[#72336174] Add a validation schema for `vcloud-net-launch`

Schema is based on these sources:

VMWare documentation:
http://pubs.vmware.com/vcd-51/index.jsp?topic=%2Fcom.vmware.vcloud.api.reference.doc_51%2Fdoc%2Findex.html

The 'orgVdcNetwork' generator in Fog:
https://github.com/fog/fog/blob/master/lib/fog/vcloud_director/generators/compute/org_vdc_network.rb

vCloud Core's `org_vdc_network.rb`:
https://github.com/gds-operations/vcloud-core/blob/master/lib/vcloud/core/org_vdc_network.rb

* * *

Also DRY up some tests.