Comment 1 for bug 1645043

I would guess this is a case where something like "cloud-init" needs python-yaml, and so most places where we install already have it available. If this is a manual machine, then who knows what packages are already installed on it.

Having the charm clearly add a "apt install python-yaml" step seems reasonable. Most likely this is just an unnoticed dependency because all other target environments already have it installed.