mantle/kola: allow rerun success for platform machine start failure

In this case it's usually the platform having some internal failure.
If it succeeds in the rerun then just be happy with that.

mantle/kola/harness.go

@@ -1654,6 +1654,9 @@ func runTest(h *harness.H, t *register.Test, pltfrm string, flight platform.Flig
 			return err
 		})
 		if err != nil {
+			// The platform failed starting machines, which usually isn't *CoreOS
+			// fault. Maybe it will have better luck in the rerun.
+			markTestForRerunSuccess(t, "Platform failed starting machines.")
 			h.Fatalf("Cluster failed starting machines: %v", err)
 		}
 	}