Revert "Remove unused ASN numbers from CloudNAT to avoid provider errors"

[LucaPrete]

Turns out that ASN for CloudNAT was everything but useless.

• the net-cloudnat module sets router_asn to 64514 by default, if not manually set.
• 64514 goes in conflict with other router asn numbers set in different FAST stages (i.e. 2-networking-e-bgp), causing errors

I tried to reproduce what a user reported in the past (below):

Error: Attribute must be a whole number, got 4.200001024e+09
│   with module.prod-spoke-cloudnat["europe-west1"].google_compute_router.router[0],
│   on ../../../modules/net-cloudnat/[main.tf](http://main.tf/) line 32, in resource "google_compute_router" "router":
│   32:     asn = var.router_asn

Anyway, it doesn't seem to be reproducible.
I've also checked the APIs and the ASN 4200001024 should be definitely allowed.

I propose to revert this to fix the networking-e stage immediately. We may otherwise consider to use lower private ASN but not conflicting with ASN numbers used in FAST already (i.e. 65534?)