Simplify kube private network example

[scraly]

In https://github.com/ovh/public-cloud-examples/blob/main/network/private-network/kube-private-network ,
don't use openstack provider if not needed and update the example with TF providers new features and resources.

[scraly]

Moreover, a public_cloud_gateway TF resource have been created in the OVhcloud TF provider.
Maybe it can be a good idea to use it also ;)