slapos_cloud: use full "Compute Node Reference" term

We don't use abbreviations in the user interface, this also makes
another message to translate.

master/bt5/slapos_cloud/SkinTemplateItem/portal_skins/slapos_cloud/InstanceTree_view/listbox.xml

@@ -141,7 +141,7 @@
                         </tuple>
                         <tuple>
                           <string>aggregate_parent_reference</string>
-                          <string>Compute Node Ref.</string>
+                          <string>Compute Node Reference</string>
                         </tuple>
                         <tuple>
                           <string>aggregate_title</string>

[Jérome Perrin]

/cc @rafael @romain

I don't think it was OK to use an abbreviation here, but I could not find the guideline where we abbreviations are not allowed in the user interface. We have https://www.erp5.com/documentation/developer/guideline/coding/erp5-Guideline.Coding.Best.Practice#abbreviations-are-not-allowed which apply for code, but for module I don't find it. This was the only place where this term is used in this repository