Fix an error on empty list from chef search

Change-Id: Iacfd5614849b3885c5be833122d1f640c34ce66c

chef/cookbooks/openstack-identity/libraries/default.rb

@@ -27,10 +27,15 @@ def node_election(role, tag, chef_environment = nil)
     nodes = search(:node, "run_list:role\\[#{role}\\] AND \
                    chef_environment:#{chef_environment}") || []
     nodes = nodes.sort_by { |node| node.name } unless nodes.empty?
-    if node.name.eql?(nodes.first.name)
+    if nodes.empty? or node.name.eql?(nodes.first.name)
       node.tags << tag unless node.tags.include?(tag)
       node.save
     end
+
+    if nodes.empty?
+      return node
+    end
+
     return nodes.first
   else
     return master.first