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

19144:16:27

running ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

19144:16:19

running SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

19144:16:00

running SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

19144:16:30

running SlapOS.Eggs.UnitTest-Master.Python3

19144:15:55

running SlapOS.KVM.ResilienceTest-Master

19144:15:55

running Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

19144:16:13

running Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch

19144:15:49