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

21713:56:10

running ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

21713:56:02

running SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

21713:55:44

running SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

21713:56:14

running SlapOS.Eggs.UnitTest-Master.Python3

21713:55:39

running SlapOS.KVM.ResilienceTest-Master

21713:55:39

running Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

21713:55:57

running Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch

21713:55:32