drush aegir refactoring

A drush update broke our cherished and fragile provision module on HEAD, which depends so dearly on drush.

So do not update your drush checkout or at least revert this patch otherwise you will not be able to install new sites in your main platform.

If you have updated your drush checkout by mistake, you can revert back to the known working date using the following command:

Provision will be synced with drush once it stabilizes.