You probably only want this cartridge if you want to run Drupal 8 on OpenShift.
Drush for Drupal 7 can be included by putting the following in depslist.txt of a PHP cartridge.
pear.drush.org/drush-6.0.0
The initial import of Drush from master was done with this command:
git subtree add --prefix usr/drush git@github.com:drush-ops/drush.git master --squash
You can pull in new changes from Drush master with the following command:
git subtree pull --prefix usr/drush git@github.com:drush-ops/drush.git master --squash