Use yaml.safe_load instead of yaml.load

We currently use yaml.load to read a user-written config file. This can
lead to malicious code execution, so we should use yaml.safe_load
instead.

Found using bandit.

Change-Id: I27792f0435bc3cb9b9d31846d07a8d47a1e7679d

oslo_messaging/notify/_impl_routing.py

@@ -61,7 +61,7 @@ class RoutingDriver(notifier.Driver):
             return
 
         # Infer which drivers are used from the config file.
-        self.routing_groups = yaml.load(
+        self.routing_groups = yaml.safe_load(
             self._get_notifier_config_file(filename))
         if not self.routing_groups:
             self.routing_groups = {}  # In case we got None from load()

oslo_messaging/tests/notify/test_notifier.py

@@ -412,7 +412,7 @@ group_1:
           - blah.zoo.*
           - zip
         """
-        groups = yaml.load(config)
+        groups = yaml.safe_load(config)
         group = groups['group_1']
 
         # No matching event ...
@@ -443,7 +443,7 @@ group_1:
           - info
           - error
         """
-        groups = yaml.load(config)
+        groups = yaml.safe_load(config)
         group = groups['group_1']
 
         # No matching priority
@@ -476,7 +476,7 @@ group_1:
       accepted_events:
           - foo.*
         """
-        groups = yaml.load(config)
+        groups = yaml.safe_load(config)
         group = groups['group_1']
 
         # Valid event, but no matching priority