fix for tty error

while running micro-qa with a centos 6.5 image, I have hit the following error,

```
An error occurred while executing multiple actions in parallel.
Any errors that occurred are shown below.

An error occurred while executing the action on the 'default'
machine. Please handle this error then try again:

The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

mkdir -p '/vagrant'

Stdout from the command:

Stderr from the command:

sudo: sorry, you must have a tty to run sudo
```

Adding the `config.ssh.pty` fixes the issue.

This was addressed in vagrant 1.4 [1].

[1] https://github.com/mitchellh/vagrant/issues/1482