Enable dynamic configuration for "xds-grpc" cluster

*Description*:
In Envoy with Istio scenario, the back end xDS server Pilot is configured through "envoy_bootstrap_v2.json" like this:
```yaml
{
"name": "xds-grpc",
"type": "STATIC",
"dns_lookup_family": "V4_ONLY",
"connect_timeout": "{{ .connect_timeout }}",
"lb_policy": "RANDOM",
...
"hosts": {{ .pilot_grpc_address_group }},
...
}
```
However, this is a static address and can't not be changed any longer. If we want to migrate the Pilot address to another, the Envoy has to reboot to reload the new Pilot address. I'm wandering if there is any way to dynamically change this address. Maybe by enabling configure file watching and reloading is a good practice.