network: Fabric.InternetNAT should have a zero value of true

[jwreagor]

Ref: terraform-providers/terraform-provider-triton#100

It's up for debate whether or not we should also create a fabric network with internet NAT by default from within triton-go. CloudAPI says it defaults to true but I'm not certain if that means we can simply omitempty for the field and let CloudAPI create the internet NAT, or if that's even viable with our current Network.Fabric.Create API and lack of zero value handling.

Either way, we should explore treating this identical to CloudAPI.

[kwilczynski]

Related to #122.

[jen20]

The reason this was different in the first place (certainly in Terraform, and likely here too as a result) is that you couldn't delete a NAT once it exists without support input. If this is still the case I am heavily in favour of leaving this alone.

[twhiteman]

NAT zones are now cleaned up when there are no more consumers of that NAT - i.e. when the last VM using the NAT is deleted, then the NAT is also deleted.

c.f. https://smartos.org/bugview/NAPI-258