Code review comment for ~raharper/curtin:feature/enable-storage-vmtest-on-centos

2 questions

a.)
 'yum update' versus 'yum upgrade'

https://unix.stackexchange.com/questions/55777/in-centos-what-is-the-difference-between-yum-update-and-yum-upgrade

this feels like we want 'upgrade' as it is more similar to 'dist-upgrade' which is what we do in apt.

b.) I think really we still want the 2 phase for upgrade.
 retry this: yum --downloadonly --setopt=keepcache=1 upgrade
 then run this: yum upgrade --cacheonly --downloadonly --setopt=keepcache=1

It looks like we can mostly re-use the existing 'yum_install' but just manage to set ['install'] to be ['upgrade']

We are getting there...
I know that this 'upgrade' path isn't a huge thing, but if we have it there i'd like for it to work reliably.