CentOS 7 uses Python 2. Although it's possible to install Python 3
on it, it not trivial with DIB. We should rather switch to
a distribution where Python 3 is the default version.
As a nice side effect, the CentOS 8 images seem smaller.
The CentOS 7 job definitions are kept around since they're used
on stable/train.
Change-Id: I6b23da21eab19c69f7b070adff76b35e1752bf8d