Use CentOS Stream 8 instead of CentOS Linux 8

CentOS Linux 8 went End Of Life at the end of 2021 and at the end of
January it will disappear from the mirrors. Stream is the supported
replacement. By using the Vagrant box and container image it should mean
most testing will not notice the difference.

lib/beaker-hostgenerator/data.rb

@@ -297,7 +297,11 @@ def osinfo
           :general => {
             'platform' => 'el-8-x86_64'
           },
+          :vagrant => {
+            'box' => 'centos/stream8',
+          },
           :docker => {
+            'image'                 => 'quay.io/centos/centos:stream8',
             'docker_image_commands' => [
               'cp /bin/true /sbin/agetty',
               'yum install -y crontabs initscripts iproute openssl wget which glibc-langpack-en'