Fix race in healthchecking etcds leading to crashes

Kubernetes-commit: 38387aec0db3eda3a7debb4558a223ac92a41389

pkg/server/storage/storage_factory.go

@@ -321,8 +321,10 @@ func (s *DefaultStorageFactory) Backends() []Backend {
 	backends := []Backend{}
 	for server := range servers {
 		backends = append(backends, Backend{
-			Server:    server,
-			TLSConfig: tlsConfig,
+			Server: server,
+			// We can't share TLSConfig across different backends to avoid races.
+			// For more details see: http://pr.k8s.io/59338
+			TLSConfig: tlsConfig.Clone(),
 		})
 	}
 	return backends