Deprecate using cloud provider to set host address feature

Long term plan is to remove all uses of cloud provider from kube api
server. As part of that, we need to remove the dependency on
figuring out the host address of the node running the kube api server
using the cloud provider. In this review, we log a warning that this
feature that is usually used for example with swagger generation
will go away in the future.

pkg/kubeapiserver/options/cloudprovider.go

@@ -81,6 +81,8 @@ func (s *CloudProviderOptions) DefaultExternalHost(genericoptions *genericoption
 			for _, addr := range addrs {
 				if addr.Type == v1.NodeExternalIP {
 					genericoptions.ExternalHost = addr.Address
+					glog.Warning("[Deprecated] Getting host address using cloud provider is " +
+						"now deprecated. Please use --external-hostname explicitly")
 				}
 			}
 		}