qemu: disable Ceph when buildout with Python 2

Some Py2-SR like neoppod/stress-testing need Qemu but not Ceph.

Also note that the conditional [systemd-markupsafe-download] only works
if [systemd-python] uses the same Python as the one that runs buildout.
14 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:05:25

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

15:09:04

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:03:24

failed ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

15:03:14

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

15:09:27

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

15:03:03

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:34

passed SlapOS.Eggs.UnitTest-Master.Python3

00:17:02

passed SlapOS.SlapRunner.ResilienceTest-Master

00:23:04

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:29:02

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

06:38:38

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

07:21:36

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

15:03:27

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch

15:03:07