prevent clusters with an even number of nodes

Clusters must have an odd number of nodes to ensure quorum during
partitioning.  2-node clusters are allowed as a special exception
for the typical DRBD setup.

crowbar_framework/app/models/pacemaker_service.rb

@@ -460,6 +460,12 @@ def validate_proposal_after_save proposal
     elements = proposal["deployment"]["pacemaker"]["elements"]
     members = elements["pacemaker-cluster-member"] || []
 
+    if members.length % 2 == 0 && members.length > 2
+      validation_error \
+        "Clusters must have an odd number of nodes to ensure quorum during " \
+        "partitioning.  2-node clusters are allowed as a special exception."
+    end
+
     if elements.has_key?("hawk-server")
       elements["hawk-server"].each do |n|
         @logger.debug("checking #{n}")