Add PyYAML dependency to cookiecutter venv

Change-Id: I92e546b7791554043e2d78fc5d268b121c0c20ed
diff --git a/src/com/mirantis/mk/Python.groovy b/src/com/mirantis/mk/Python.groovy
index 4195c66..eea0fd7 100644
--- a/src/com/mirantis/mk/Python.groovy
+++ b/src/com/mirantis/mk/Python.groovy
@@ -217,7 +217,8 @@
 def setupCookiecutterVirtualenv(path) {
     requirements = [
         'cookiecutter',
-        'jinja2==2.8.1'
+        'jinja2==2.8.1',
+        'PyYAML==3.12'
     ]
     setupVirtualenv(path, 'python2', requirements)
 }