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
Use `let` rather than `let!` in Rspec tests

After discussion with @dancarley (who committed this code in
96441e5f6d26778f983dcdc63efd7c1df8638ba2), I can't see any reason for
these `let!` declarations not to lazy-load using `let` (without the
trailing exclamation mark).

The tests also pass without the exclamation mark.
DRY up spec tests using `let`
IP addresses should be validated as such
Add tests for the vcloud-net-launch schema

Add some simple tests for the Vcloud::NetLauncher::Schema::NET_LAUNCH
schema, used by the `vcloud-net-launch` command.
Merge pull request #34 from gds-operations/72336174-validation-schema

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