Replace yaml.load() with yaml.safe_load()

Avoid dangerous file parsing and object serialization libraries.
yaml.load is the obvious function to use but it is dangerous[1]
Because 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.

In addition, Bandit flags yaml.load() as security risk so replace
all occurrences with yaml.safe_load(). Thus I replace yaml.load()
with yaml.safe_load()

[1]https://security.openstack.org/guidelines/dg_avoid-dangerous-input-parsing-libraries.html

Change-Id: I4aac907c2a7ecf9a3f85a1a07ca020df6eb0b756
Closes-Bug: #1634265

rally/plugins/openstack/verification/tempest/manager.py

@@ -121,7 +121,7 @@ class TempestManager(testr.TestrLauncher):
                 "Cannot list installed Tempest plugins for verifier %s." %
                 self.verifier)
 
-        return yaml.load(output)
+        return yaml.safe_load(output)
 
     def uninstall_extension(self, name):
         """Uninstall a Tempest plugin."""