MySQL software release : try to make the 'the directory couscous used as download cache does not exist'

MySQL software release : Add egg part so that buildout executes the git clone before looking for cookbook

Follow the rules. Do not add new things into 'master' branch, but use
slapos one. It was used to introduce slapos component, and as repository
is about slapos, this place seems the best way to start explaining.

This reverts commit 01d10d6c. (slapos.reboostrap name was good)