Skip failing test for now

This test fails randomly on VMs, there's an issue to fix this so
skipping it for now

ca/bootstrap_test.go

@@ -370,6 +370,7 @@ func TestBootstrapClient(t *testing.T) {
 }
 
 func TestBootstrapClientServerRotation(t *testing.T) {
+	t.Skipf("skip until we fix https://github.com/smallstep/certificates/issues/873")
 	reset := setMinCertDuration(1 * time.Second)
 	defer reset()