Add note about VAGRANT_PREFERRED_PROVIDERS for first vagrant up

For users on certain Linux systems without VirtualBox installed (but with something like libvirt), the first `vagrant up` (around pages 10-11) can result in a bunch of misleading error messages.

From reader 'T':

> Can I ask, therefore, that you add an additional step on pages 10/11 before item "6) Boot your CentOS server: vagrant up", that asks the readers to ensure either:
>
> The user sets their environment with the line :
>
> ```
> export VAGRANT_PREFERRED_PROVIDERS=virtualbox
> ```
>
> or ensure that `/etc/default/vagrant` has an entry of :
>
>
> ```
> VAGRANT_PREFERRED_PROVIDERS=virtualbox
> ```
>
> Once that is set up 'vagrant up' works perfectly.

Review the instructions around pages 10-11, immediately before item 6, “Boot your CentOS server: vagrant up.” Add a note explaining the preferred-provider setting requested in the issue, then verify that the first-up procedure clearly tells users how to avoid the misleading errors.

Written by the indexing model from the issue text.