Add drush for Backdrop CMS integration.

[klonos]

In order for drush to work with Backdrop CMS, 2 things are required:

1. The Drush Integration for Backdrop CMS set of drush commands (needs to be copied/cloned under /usr/local/share/drush/commands/backdrop in the VM).
2. Drush v8:

This project requires that you use the "8.x" branch of drush (https://github.com/drush-ops/drush/tree/8.x). Neither older versions of drush nor the new 9.x or master branches will work with this extension
...

https://github.com/backdrop-contrib/drush#installation