Drupal task updates

Thanks for your merges earlier. These commits relate to issues I've been having related to Vagrant, GitHub and  NFS support. They make it so that:

    - The repo only gets checked out if it doesn't exist.
    - The hostkey is automatically accepted.
    - Drush has sufficient privileges to create the settings.php file (per info at https://www.drupal.org/documentation/install/settings-file).
    - The site only gets installed if there is not yet a settings.php file.

Thanks Jeff. Please let me know if you have questions or concerns.