tests: use CentOS 8.4 image

CentOS 8.4 vagrant image is available at https://cloud.centos.org
let's use it.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit c2aaa96)

tests/scripts/vagrant_up.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 
 vagrant box remove --force --provider libvirt --box-version 0 centos/8 || true
-vagrant box add --provider libvirt --name centos/8 https://cloud.centos.org/centos/8/vagrant/x86_64/images/CentOS-8-Vagrant-8.3.2011-20201204.2.x86_64.vagrant-libvirt.box || true
+vagrant box add --provider libvirt --name centos/8 https://cloud.centos.org/centos/8/vagrant/x86_64/images/CentOS-8-Vagrant-8.4.2105-20210603.0.x86_64.vagrant-libvirt.box || true
 
 retries=0
 until [ $retries -ge 5 ]