- pass parameter directly instead of indirectly via class
- call it simply "images", to harmonize with clean-function later
- harmonize to svr_id as in the previous commit
- rewrite function to use with-statement with cls_cmd
- optionally pass a tag to use (which will be used in a test, and can be
  used to pull flexric)
- Introduce Pull_Image() which does the actual pulling, can pull from
  any registry, and will be reused in the case of pulling from cluster.