Add param docstring into the normalize func

This patch adds a missing param docstring into the normalize function
in util.py.

Change-Id: I91ecc0970fc386163657ab521d8cdd898694a426
Partial-bug: 1367915

ironic_python_agent/utils.py

@@ -218,6 +218,7 @@ def normalize(string):
 
     Take a urlencoded value from Ironic and urldecode it.
 
+    :param string: a urlencoded string
     :returns: a normalized version of passed in string
     """
     return parse.unquote(string).lower().strip()