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.
1 job for fix/glib-python
Status Job ID Name Coverage
  External
passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch