Kubelet tests broken on Macs with uppercase names

Hostname behavior across operating systems is inconsistent (Macs can
have uppercase host names, so can some other systems).  In general,
always strings.ToLower(os.Hostname()).

pkg/util/node.go

@@ -34,5 +34,5 @@ func GetHostname(hostnameOverride string) string {
 		}
 		hostname = fqdn
 	}
-	return strings.TrimSpace(string(hostname))
+	return strings.ToLower(strings.TrimSpace(string(hostname)))
 }