Use more secure yaml.safe_load() instead of yaml.load()

The function yaml.load() provides the ability to construct an arbitrary
Python object. For security, we use yaml.safe_load() instead which
limits this ability to simple Python objects(like integers or lists).

ref: https://en.wikipedia.org/wiki/YAML#Security

Change-Id: Iea2a2d79c764d635b02c5d6d36c9a5652010d716

tools/simulator.py

@@ -54,7 +54,7 @@ Usage example:
 def init_random_generator():
     data = []
     with open('./messages_length.yaml') as m_file:
-        content = yaml.load(m_file)
+        content = yaml.safe_load(m_file)
         data += [int(n) for n in content[
             'test_data']['string_lengths'].split(', ')]