Use libyaml if possible

PyYAML doesn't automatically use the much faster and more memory
efficient libyaml bindings, even if the extension is available. So we
provide our own module that exports the pieces needed to use the faster
one, or fall back to the pure python implementation.

Change-Id: I7ee99f5017cb83153ab8fa9bc23548ed639777c1
diff --git a/tests/unit/test_model.py b/tests/unit/test_model.py
index f906095..2167a3b 100644
--- a/tests/unit/test_model.py
+++ b/tests/unit/test_model.py
@@ -18,11 +18,11 @@
 
 import fixtures
 import testtools
-import yaml
 
 from zuul import model
 from zuul import configloader
 from zuul.lib import encryption
+from zuul.lib import yamlutil as yaml
 
 from tests.base import BaseTestCase, FIXTURE_DIR