GCE: support egress specification

[justinsb]

Empty or "nat" now defaults to creating a per-subnet NAT router for
private topologies. "external" will assume that egress is configured
outside of kOps.

Issue #12563

[zetaab]

@justinsb overall looks good, please fix the terraform part and good to go

upup/pkg/fi/cloudup/gcetasks/router.go:231:31: subnet.TerraformLink undefined (type *Subnet has no field or method TerraformLink)

[rifelpet]

Suggested change

	Name: subnet.TerraformLink(),
	Name: subnet.TerraformName(),

kops/upup/pkg/fi/cloudup/gcetasks/subnet.go

Lines 238 to 240 in bfaee4e

	func (i *Subnet) TerraformName() *terraformWriter.Literal {
	return terraformWriter.LiteralProperty("google_compute_subnetwork", *i.Name, "name")
	}

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[rifelpet]

prow job pod timeout :(

/retest

[rifelpet]

I wonder if the pod is being OOMkilled. the job doesn't have a memory limit in place

/retest