Route Replacement does not work with UpstreamGroups

[lgadban]

Describe the bug
Invalid route replacement does not work for routes that refer to a nonexistent UpstreamGroup

To Reproduce

1. Create a route to refer to an UpstreamGroup and confirm it works
2. Enable route replacement, e.g. --

    invalidConfigPolicy:
      invalidRouteResponseBody: Gloo Gateway has invalid configuration. Administrators
        should run `glooctl check` to find and fix config errors.
      invalidRouteResponseCode: 404
      replaceInvalidRoutes: true

3. See error --

[2020-11-04 16:07:42.886][8][debug][misc] [external/envoy/source/common/protobuf/utility.cc:251] Proto validation error;
throwing Proto constraint validation failed (RouteConfigurationValidationError.VirtualHosts[i]: ["embedded message failed 
validation"] | caused by VirtualHostValidationError.Routes[i]: ["embedded message failed validation"] | caused by 
RouteValidationError.Route: ["embedded message failed validation"] | caused by field: "cluster_specifier", reason: is 
required): name: "listener-::-8080-routes"

followed by a dump of envoy config, specifically the route for the missing UpstreamGroup will be missing a cluster definition:

virtual_hosts {
  name: "gloo-system_petstore"
  domains: "*"
  routes {
    match {
      prefix: "/echo2"
    }
    route {
      prefix_rewrite: "/"
    }
  }
  routes {
    match {
      prefix: "/"
    }
    route {
      cluster: "default-petstore-8080_gloo-system"
    }
  }
  per_filter_config {
    key: "envoy.filters.http.ext_authz"
    value {
      fields {
        key: "disabled"
        value {
          bool_value: true
        }
      }
    }
  }
}

VirtualService snippet:

spec:
  virtualHost:
    domains:
    - '*'
    routes:
    - matchers:
      - prefix: /echo2
      options:
        prefixRewrite: /
      routeAction:
        upstreamGroup:
          name: my-service-group
          namespace: gloo-system
    - matchers:
      - prefix: /
      routeAction:
        single:
          upstream:
            name: default-petstore-8080
            namespace: gloo-system
status:
  reason: "warning: \n  Route Warning: InvalidDestinationWarning. Reason: *v1.UpstreamGroup
    {my-service-group gloo-system} not found\nRoute Warning: InvalidDestinationWarning.
    Reason: *v1.UpstreamGroup {my-service-group gloo-system} not found"
  reported_by: gateway
  state: 3
  subresource_statuses:
    '*v1.Proxy.gloo-system.gateway-proxy':
      reason: "warning: \n  Route Warning: InvalidDestinationWarning. Reason: *v1.UpstreamGroup
        {my-service-group gloo-system} not found\nRoute Warning: InvalidDestinationWarning.
        Reason: *v1.UpstreamGroup {my-service-group gloo-system} not found"
      reported_by: gloo
      state: 3

Expected behavior
Route replacement should work for UpstreamGroups

Additional context

% glooctl version
----------
glooctl binary version (1.5.3) differs from server components (v1.4.13) by at least a minor version.
Consider running:
glooctl upgrade --release=v1.4.13
----------

Client: {"version":"1.5.3"}
Server: {"type":"Gateway","enterprise":true,"kubernetes":{"containers":[{"Tag":"1.4.16","Name":"grpcserver-ui","Registry":"quay.io/solo-io"},{"Tag":"1.4.16","Name":"grpcserver-ee","Registry":"quay.io/solo-io"},{"Tag":"1.4.16","Name":"grpcserver-envoy","Registry":"quay.io/solo-io"},{"Tag":"1.4.13","Name":"discovery","Registry":"quay.io/solo-io"},{"Tag":"1.4.16","Name":"extauth-ee","Registry":"quay.io/solo-io"},{"Tag":"1.4.13","Name":"gateway","Registry":"quay.io/solo-io"},{"Tag":"1.4.16","Name":"gloo-ee-envoy-wrapper","Registry":"quay.io/solo-io"},{"Tag":"1.4.16","Name":"gloo-ee","Registry":"quay.io/solo-io"},{"Tag":"1.4.16","Name":"rate-limit-ee","Registry":"quay.io/solo-io"},{"Tag":"5","Name":"redis","Registry":"docker.io"}],"namespace":"gloo-system"}}