Unify DefaultNetworkDevice and AdditionalNetworkDevices

[65278]

Describe the solution you'd like
At the moment, Network devices are arbitrarily split in two categories, default and additional. This duplicates code and serves no purpose.
If we just have an array of NetworkDevices, and require there to be at least 1, we get the same guarantees.

Anything else you would like to add:

• IP addresses for the first network device are a problem, because they're coming from the cluster object. This needs some architecture.
• This changes public API. We'll need to go v1alpha2 and provide migration patches.

[mcbenjemaa]

We need to be aware of network devices names,
for instance, the first device must have the name net0

[wikkyk]

We could stop creating a default gw for every interface at the same time.

[wikkyk]

ref #51