Generated VirtualService contains wrong gateways field

[yanniszark]

The gateways field of the VirtualService CRD indicates to which Istio
Gateways this VirtualService applies to.

At the moment, all configured gateways of are added to the gateways fields.
However, if the user wants to only specify a local gateway, they also
get the public gateway in that field.

This confuses the reconciler, which can't find the gateway and errors out.
The VirtualService generation logic should instead only add gateways which are actually used by the VirtualService routes.

Related code: https://github.com/knative/net-istio/blob/f08de598438b0e85c27b401416575405cc451eba/pkg/reconciler/ingress/resources/virtual_service.go#L135-L140
Related issue: #43
/cc @tcnghia