Fix or verify the GC logic for multi-cluster ingress

[MrHohn]

Ref #825 (comment), we may have broke the GC logic for multi-cluster ingress --- we need to keep the instance group if we have at least one multi-cluster ingress. The current logic may delete the instance group if there isn't any gce ingresses (regardless of multi-cluster ingress).

(By multi-cluster ingress I meant below)

ingress-gce/pkg/utils/utils.go

Lines 288 to 293 in 78801b3

	// IsGCEMultiClusterIngress returns true if the given Ingress has
	// ingress.class annotation set to "gce-multi-cluster".
	func IsGCEMultiClusterIngress(ing *v1beta1.Ingress) bool {
	class := annotations.FromIngress(ing).IngressClass()
	return class == annotations.GceMultiIngressClass
	}

/assign @skmatti

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In response to this:

Ref #825 (comment), we may have broke the GC logic for multi-cluster ingress --- we need to keep the instance group if we have at least one multi-cluster ingress. The current logic may delete the instance group if there isn't any gce ingresses (regardless of multi-cluster ingress).

(By multi-cluster ingress I meant below)

ingress-gce/pkg/utils/utils.go

Lines 288 to 293 in 78801b3

	// IsGCEMultiClusterIngress returns true if the given Ingress has
	// ingress.class annotation set to "gce-multi-cluster".
	func IsGCEMultiClusterIngress(ing *v1beta1.Ingress) bool {
	class := annotations.FromIngress(ing).IngressClass()
	return class == annotations.GceMultiIngressClass
	}

/assign @skmatti

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