vSphere: do not add new network adapters by default

[mkuzmin]

An the moment a new network adapter is added every time a virtual machine is created.
A quote from https://github.com/rakutentech/terraform-provider-vsphere/blob/master/README.md:

please don't include network adapters in VM template. When deploying a new virtual machine, this provider adds new network adapters to the new virtual machine.

This requirement is fine, if a machine is created from scratch, and then loaded from PXE. But that's not the main use case.
More often machines are cloned from templates, and here such requirement makes the procedure extremely complicated. To make my templates compatible to Terraform, I have to remove network adapters explicitly. But what if later I need to update such a template? I have to add a network adapter back, load a machine, make required changes, and then delete the adapter once again. That just doesn't work.

If Terraform clones a virtual machine from a template, then network_interface parameter must be optional. By default it should inherit configuration from a template, and do not create additional network adapters.

[ghost]

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