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_n310_2X2_100MHz_quectel.xml

@@ -46,8 +46,8 @@
   <testCase id="111111">
     <class>Pull_Cluster_Image</class>
     <desc>Pull Images from Cluster</desc>
-    <images_to_pull>oai-gnb</images_to_pull>
-    <test_svr_id>matix</test_svr_id>
+    <images>oai-gnb</images>
+    <node>matix</node>
   </testCase>
 
   <testCase id="100000">