live base: Migrate yum's yumdb over to dnf
As livecd-creator is still yum based, we only get yum's yumdb during
live image composes. To work this around, this commit adds a %post
script to fedora-live-base.ks to migrate yum's yumdb over to dnf.
https://bugzilla.redhat.com/show_bug.cgi?id=1274319