use the recommended yaml.safe_load function

reason: with pyyaml >= 5, the load function without Loader is deprecated.
Even if we do not use version 5 for now, we should use good practices.
https://pyyaml.org/wiki/PyYAMLDocumentation#loading-yaml
yaml/pyyaml#257

xivo_confd/config.py

@@ -195,7 +195,7 @@
 def load(argv):
     try:
         with open(WIZARD_KEY_FILE, 'r') as f:
-            key_config = {'wizard': yaml.load(f)}
+            key_config = {'wizard': yaml.safe_load(f)}
     except IOError:
         key_config = {}