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: I4b554fa20c2391a0f7a11fd5d6f5f944f0de2d03
(cherry picked from commit ebe583f558a347800e55d7f4832fc2bc497bee60)

timmy/tools.py

@@ -255,7 +255,7 @@ def load_yaml_file(filename):
     """
     try:
         with open(filename, 'r') as f:
-            return yaml.load(f)
+            return yaml.safe_load(f)
     except IOError as e:
         logger.critical("I/O error(%s): file: %s; msg: %s" %
                         (e.errno, e.filename, e.strerror))