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

wazo_plugind/db.py

@@ -1,4 +1,4 @@
-# Copyright 2017-2018 The Wazo Authors  (see the AUTHORS file)
+# Copyright 2017-2019 The Wazo Authors  (see the AUTHORS file)
 # SPDX-License-Identifier: GPL-3.0-or-later
 
 import logging
@@ -264,7 +264,7 @@ def is_installed(self, version=None):
     def metadata(self):
         if not self._metadata:
             with open(self.metadata_filename, 'r') as f:
-                self._metadata = yaml.load(f)
+                self._metadata = yaml.safe_load(f)
 
         return self._metadata