Run database update after rebuilding the site
Remove unnecessary requirements
Extract drush commands into variables
User running the Drupal site is configurable
Move default config directory to /etc/drush

This is the default configuration directory used by drush itself.
Expose composer configuration parameters
Make location of the drush executable configurable
Use drush_path instead of just drush

This should prevent the need to put drush into the PATH
Replace composer_install_dir with composer_path
Drush version is configurable