CentOS 7 is the only distro we support currently that doesn't have
Python 3 installed in some form in the base images. For centos 7 add
an early install of it in the yum element so we can have all the
in-chroot scripts assume Python 3. There is only one package that
causes issues; yaml which comes from EPEL. Everywhere else it is a
base package, but we don't have a way to say "enable epel to install
this". Just hack it in, we don't want to go reworking the world for
CentOS 7 at this point.
Also add python3 and it's yaml library to the centos 8 path. This
brings in the "user" python3 in /urs/bin/python3 (the "system" python3
is already installed). Again, this just lets us assume
/usr/bin/python3 in scripts for all platforms.
package-installs is one of these things running python in the chroot,
and unfortunately we have elements that use it at 01- level in
pre-installd. Thus to make sure python3 is there nice and early, run
it at 0 level, but make sure it comes after yum/dnf update.
Change-Id: I088fc4284e889147ca9a375d4a159264cff53484
