Supporting MFA

[joelio]

Hi, thanks for the plugin firstly!

Hitting an issue when using MFA and assume role stuff

➜  git:(vagrant_s3) ✗ vagrant up
Bringing machine 'app1' up with 'virtualbox' provider...
==> app1: Box 'app1' could not be found. Attempting to find and install...
    app1: Box Provider: virtualbox
    app1: Box Version: >= 0
Unable to communicate with Amazon S3 to download box. The S3 API reports:

    MultiFactorAuthentication failed, must provide both MFA serial number and one time pass code.

I'll take a look at a fix and PR, but raising here for now to track