Title: Support api_config_source without cluster_names

Description:
To support deprecation of cluster_names in api_config_source (https://github.com/envoyproxy/envoy/blob/master/source/common/config/utility.cc#L184)

we should relax/remove assertions that cluster_names.size() is 1 (https://github.com/envoyproxy/envoy/blob/master/source/common/config/utility.cc#L185)

and also handle call sites that extract the cluster name field value (https://github.com/envoyproxy/envoy/blob/master/source/server/lds_subscription.cc#L20)

For example, the following LDS config which does not have field cluster_names should be considered valid:

          lds_config {
            api_config_source {
              api_type: GRPC
              grpc_services {
                google_grpc {
                  target_uri: "10.0.0.1:80"
                  stat_prefix: "abcde"
                  config {
                  }
                }
              }
            }
          }