I could workaround the issue by manually downgrading some rpms . Luckily we have a epel repo mirror where we don’t delete any file.
In case it helps for anyone else I have uploaded here is the list of rpms I had to downgrade: http://barrabin.org/katello-3.5-custom-rpms/
you can download all those rpms then do yum localinstall *rpm and use
"yum versionlock" ( https://www.systutorials.com/docs/linux/man/1-yum-versionlock/) to prevent those packages to be upgraded.
After this I can do yum install katello and then run the foreman-installer