hardcoded_password_default fails on python2 comparing with ast.arg

[ehooo]

Describe the bug
hardcoded_password_default check with ast.arg on python2

To Reproduce
Steps to reproduce the behaviour:

1. Run bandit with python2
2. Create a file with:

def my_funct((a, b)):
    pass

Expected behaviour
Check the python version before check with ast.arg

Bandit version

bandit 1.5.1
  python version = 2.7.12 (default, Dec  4 2017, 14:50:18) [GCC 5.4.0 20160609]

Traceback

[tester]        ERROR   Bandit internal error running: hardcoded_password_default on file test.py at line 1: 'module' object has no attribute 'arg'Traceback (most recent call last):
  File "/home/vtorre/.virtualenvs/bandit/local/lib/python2.7/site-packages/bandit/core/tester.py", line 64, in run_tests
    result = test(context)
  File "/home/vtorre/.virtualenvs/bandit/local/lib/python2.7/site-packages/bandit/plugins/general_hardcoded_password.py", line 212, in hardcoded_password_default
    if isinstance(key, ast.Name) or isinstance(key, ast.arg):
AttributeError: 'module' object has no attribute 'arg'

[ehooo]

I also fix on #387
@sigmavirus24 @ericwb @lukehinds could you take a look?
Thanks in advance.

[lukehinds]

@ehooo I think we can close this now? (fixed in #387)?