This change uses dnf to build the container image for Centos-8
using dnf locally rather than rely on an external image that is
downloaded and unpacked.
The existing image prestage commands are made conditional, and
an operating system specific command can be provided via role
variables to build a chroot in /var/lib/machines.
During the transition from Centos-8 to Centos-8-Stream, the
vars files are separated, with vars/redhat.yml covering Stream,
and vars/centos-8.3.yml covering legcay Centos-8.
In addition, the systemd-logind service is masked from the base
image. This is masked in the previously downloaded container base
image, so we ensure that the same is done for locally built chroots.
Depends-On: I31880ca995735b737d33532eaa4c29be02523117
Depends-On: I74f02669b013b8580d3469a8ffe214d88cd0f525
Change-Id: I1ddfe36259610b25e86b69d64d1d7f32a56c0e4d
791b9e813e