VHDS in Route configuration does not support ADS

[rettuna]

Title: VHDS in Route configuration does not support ads

Description:
I have only an Incremental: AggregatedDiscoveryService.DeltaAggregatedResources server configured. I want the envoy to pull the virtual hosts using the ADS. But I get this error

err:vhds: only 'DELTA_GRPC' is supported as an api_type.

My config

{
		"name": "myrouteconfig0",
		"vhds": {
			"config_source": {
				"ads": {}
			}
		}
	}

If I change the config to use DELTA_GRPC then envoy doesn't use the ADS RPC to query virtual hosts and expects an incremental VHDS host to listen for Virtual host requests.

[ravenblackx]

@dmitri-d or @htuch maybe knows stuff about vhds.

[htuch]

#12742 is the explanation I think, @dmitri-d to confirm.