In Id1e430e7d050a0b99ac449e2ea435e06cda1c4e6 I made the mistake of not
actually removing grub in 15-remove-grub.
This restores the removal phase and adds a bunch of comments. It
seems the centos7 and centos (6) images have grub2 installed, but F22
does not; hence the check.
For anyone interested in the history; it seems the whole idea of
removing grub and re-installing it in the finalise stage is to do with
Ubuntu grub scripts failing in the chroot. It is not clear this does,
or has ever, affected rpm based systems; but that's how it is, so
leave well enough alone.
The whole reasoning behind the rpm download & re-install is actually
explained in If095adc4abb52a19a3aa0b1caebfb3e4d8f605ef, but over time
the comments got lost as code moved around. I've restored in here
some detailed explaination of why we don't just re-install the package
"normally". I've also added a note to the pre-install of various
things that are related to this step. Again I think there are some
questions around this that we can investigate in another change.
Change-Id: I1acd19da8567ab93b5003caf67673cc70efea5fa
8349f0350e