remove unnecessary decoding process for mfa device creation

This fixes an issue where `aws iam create-virtual-mfa-device` tries to
base64-decode already base64-decoded data.
2nd decoding results in decoding error or malformed data.

This happened whichever `--bootstrap-method` was used.
fixes #970

API

* http://docs.aws.amazon.com/cli/latest/reference/iam/create-virtual-mfa-device.html
* http://docs.aws.amazon.com/IAM/latest/APIReference/API_CreateVirtualMFADevice.html