CI: Pull_Image_from_Registry(): harmonize code

- 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.

ci-scripts/xml_files/container_sa_e1_b200_quectel.xml

@@ -56,8 +56,8 @@
   <testCase id="333333">
     <class>Pull_Local_Registry</class>
     <desc>Pull Images from Local Registry</desc>
-    <test_svr_id>0</test_svr_id>
-    <images_to_pull>oai-gnb oai-nr-cuup</images_to_pull>
+    <svr_id>0</svr_id>
+    <images>oai-gnb oai-nr-cuup</images>
   </testCase>
 
   <testCase id="310000">