Check boot failures with custom mountpoints with edge-ami on RHEL 9.0 and 9.1

[achilleas-k]

We had issues booting RHEL 9.0 and RHEL 9.1 edge-ami images when they were built with custom mountpoints. The steps are skipped for now and it wasn't possible to reproduce locally.

I'm fine with limiting these but I'd like to investigate what's wrong with 9.0 and 9.1. If it's some weirdness that is distro/package version specific, it's probably fine, but if it's something we're doing, it might be a symptom of a bigger issue, so it would be good to know.

Originally posted by @achilleas-k in #255 (comment)

[say-paul]

The failure is ssh-keyscan is not having any output, which prevents the CI to ssh into the ec2-instance and do its testing.

[achilleas-k]

It's probably 15e9d28
RHEL 9.0 and 9.1 EC2 are BIOS only: f335096
Will be fixed as part of #318, but I'll make another PR to enable the customization tests on 9.0 and 9.1 before closing this.