Fix iam create-virtual-mfa-device to work with relative output paths.

[quiver]

http://docs.aws.amazon.com/cli/latest/reference/iam/create-virtual-mfa-device.html

If you specify output filename with relative path for iam-CreateVirtualMFADevice, you end up with write error

$ aws iam create-virtual-mfa-device --virtual-mfa-device-name test --bootstrap-method QRCodePNG --outfile QRCode.png
Unable to write to file: QRCode.png

If you specify absolute path(like /tmp/QRCode.png), this works just fine.

This is because aws checks for outfile' s write availability before it actually writes to it, and this is done through outfile's parent directory's write permission. So if you cannot restore outfile's parent path from given argument, os.access(parent_dir, os.W_OK) always fails.

[jamesls]

Looks great, thanks. Also thanks for adding tests! I'll be merging this shortly.

[jamesls]

Rebased and merged via 493399f Thanks again.