Hi Ugo,
The master must be upgraded before the agents or in other words, the
agents cannot have newer versions than the master.
The easiest and most reliable path forward is to use yum.puppetlabs.com
(ideally, your own local mirror) as the source for your packages, if you
want to support EL 5 - 7. Using the corresponding EPEL repo for each
distro is not going to work.