Replaces yaml.load() with yaml.safe_load()

Yaml.load() return Python object may be dangerous if you receive
a YAML document from an untrusted source such as the Internet.
The function yaml.safe_load() limits this ability to simple Python
objects like integers or lists.

Reference:
https://security.openstack.org/guidelines/dg_avoid-dangerous-input-parsing-libraries.html

Change-Id: I9841e688e16e9bebef5eb278ee2f3a12f2ca61f4

migration/manage_migration.py

@@ -30,7 +30,7 @@ def handle_external_config(params):
         with open(params.config) as f:
             from migration import config
 
-            content = yaml.load(f)
+            content = yaml.safe_load(f)
             if isinstance(content, dict):
                 for k, v in content.items():
                     setattr(config, k, v)