deploy-test: pre-download some images that we use in most test suites

Instead of downloading the image every time before running the test
we pre-download all images during the software installation. This
prevents downloading ~2Go every time, which is sometimes too long for
the slapos node instance step.
Status Job ID Name Coverage
  External
running Cloudooo.UnitTest-Master

14325:08:21

running ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

14325:08:13

running SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

14325:07:55

running SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

14325:08:25

running SlapOS.Eggs.UnitTest-Master.Python3

14325:07:50

running SlapOS.KVM.ResilienceTest-Master

14325:07:50

running Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

14325:08:08

running Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch

14325:07:43