slapos_pdm: Fixup in case the instance tree has 2 instances on successor_list

See merge request !604

master/bt5/slapos_pdm/SkinTemplateItem/portal_skins/slapos_pdm/InstanceTree_getNewerSofwareRelease.py

-instance_tree = context
+slap_state = ['start_requested', 'stop_requested']
+if not context.getSlapState() in slap_state:
+  return None
 
-software_instance = instance_tree.getSuccessorValue()
-if not software_instance:
+software_instance_list = [q for q in context.getSuccessorValueList() if q.getSlapState() in slap_state]
+if len(software_instance_list) == 0:
   return None
+software_instance = software_instance_list[0]
+
 software_release_list = context.SoftwareProduct_getSortedSoftwareReleaseList(
                           software_release_url=software_instance.getUrlString())