CI: PullClusterImage(): harmonize code

- reuse cls_containerize.Containerize.PullImage()
- pass parameter directly instead of indirectly via class
- always read node name and use this instead of "sometimes" looking it
  up from a class member. With "sometimes" I mean that if no node name
  is given, it will take it from self.eNBIPAddress, which is confusing
  at best if we can give the node name directly
- consistently put the node name in the xml file

ci-scripts/xml_files/container_sa_aw2s_asue.xml

@@ -65,7 +65,8 @@
     <class>Pull_Cluster_Image</class>
     <desc>Pull Images from Cluster</desc>
     <oc_project>oaicicd-ran</oc_project>
-    <images_to_pull>oai-gnb-aw2s</images_to_pull>
+    <images>oai-gnb-aw2s</images>
+    <node>avra</node>
   </testCase>
 
   <testCase id="101000">