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

@@ -510,6 +510,12 @@ def validate_proposal_after_save proposal
     elements = proposal["deployment"]["pacemaker"]["elements"]
     members = elements["pacemaker-cluster-member"] || []
 
+    if members.length.even? && members.length > 2
+      validation_error I18n.t(
+        "barclamp.#{bc_name}.validation.cluster_size_even"
+      )
+    end
+
     if elements.key?("hawk-server")
       elements["hawk-server"].each do |n|
         @logger.debug("checking #{n}")

crowbar_framework/config/locales/pacemaker/en.yml

@@ -124,3 +124,6 @@ en:
         quorum_policy: 'Invalid no-quorum-policy value: %{no_quorum_policy}.'
         platform: 'All nodes in proposal must have the same platform.'
         pacemaker_proposal: 'Nodes cannot be part of multiple Pacemaker proposals, but %{other_member} is already part of proposal \"%{p_name}\".'
+        cluster_size_even:
+          'Clusters must have an odd number of nodes to ensure quorum during
+          partitioning. 2-node clusters are allowed as a special exception.'