typo in format_str_for_message argument name

before fix:
TypeError                                 Traceback (most recent call last)
<ipython-input-29-b89509bfa590> in <module>()
----> 1 utils.format_str_for_message(sr = "nnn")

/usr/local/autotest/client/shared/utils.pyc in format_str_for_message(sr)
   2984     :param str: string that will be formatted.
   2985     """
-> 2986     lines = str.splitlines()
   2987     num_lines = len(lines)
   2988     sr = "\n".join(lines)

TypeError: descriptor 'splitlines' of 'str' object needs an argument

after fix:
In [14]: utils.format_str_for_message("nnn")
Out[14]: ' nnn'
Prefer to change the line below to sr not str. As str is special in python. And also need update the docstring.