Mysql : add explicitly installation of rdiff-backup (put out the comment so that buildout understands)

Mysql recipe : follow http://git.erp5.org/gitweb/slapos.git/commitdiff/5d0dfed4be748aaf62790d0746c4ed1ed34b1105 solution

MySQL : self.egg.working_set([__name__]) does not work : __name__ is 'slapos.recipe.mysql' but this egg does not exist so buildout fails miserably while finding a distribution 'slapos.recipe.mysql'. slapos.cookbook exists. ugly workaround.