client: consider service config invalid if loadBalancingConfig contains no supported policy

[dfawley]

If loadBalancingConfig is not specified in the service config, this has no effect.

Relevant part of gRFC:

message ServiceConfig {
...
  // Multiple LB policies can be specified; clients will iterate through
  // the list in order and stop at the first policy that they support. If none
  // are supported, the service config is considered invalid.
  repeated LoadBalancingConfig load_balancing_config = 4;
}

[dfawley]

cc @ejona86 @markdroth FYI