Running taskcat with --public_s3_bucket as the only arg causes exception

[tonynv]

jenkins@develop:workspace:[alfred] taskcat --public_s3_bucket

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| |_ __ _ | | _____ __ | |
| _/ / __| |/ / __/ _ | __|
| || (| _ \ < (| (| | |
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version 2018.410.184245
Traceback (most recent call last):
File "/var/lib/jenkins/.local/bin/taskcat", line 58, in
main()
File "/var/lib/jenkins/.local/bin/taskcat", line 28, in main
tcat_instance.set_config(args.config_yml)
File "/var/lib/jenkins/.local/lib/python3.6/site-packages/taskcat/stacker.py", line 240, in set_config
if os.path.isfile(config_yml):
File "/usr/lib64/python3.6/genericpath.py", line 30, in isfile
st = os.stat(path)
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

[tonynv]

Fixed in 3abe74e