I wasn't mistrustful enough at the beginning. One explanation is that i know Linux, but i never used Fedora before, so i didn't know the actual version number....
How could i imagine that they provide a 2 years old image ? To remind, here is what they provide:
Version 8 is old enough that you can't upgrade in one shot. You have to upgrade first to version 10 to then upgrade to version 11. I realized that trying to install nagios version 3 without being able to find it...
I found the way to upgrade through Amazon forums:
Upgrade to version 10:
Upgrade to version 11:
I highly recommend to start by upgrading Fedora version before anything else. As i realized the problem close to the end of my setup, i had packages issues after upgrading, and even some packages i had to remove before upgrading due to dependencies problems!
At least the flush privileges wasn't working anymore: