scalability_test: better guessing of distributing node

this script supported the case of a cluster with only activity family
and no distributing family, in that case one node from activity family
was used for distribution, but this node was also used for activity
processing.
This change to use the node only as distribution and not processing in
that case, because it's not recommended to use a node both for
distribution and processing

bt5/erp5_scalability_test/SkinTemplateItem/portal_skins/erp5_scalability_test/ERP5Site_setUpActivityTool.py

@@ -20,6 +20,7 @@ if distribution_node_list:
 
 if distribution_node is None:
   distribution_node = activity_node_list[0]
+  activity_node_list = activity_node_list[1:]
   # default ERP5 instanciation uses following schema for naming all zope nodes
   # activities-0..9 when number of nodes <= 10
   # activites-01..09 (or n) when number of nodes > 10