Add lans as a virtual relationship to ems_cluster

Lans need to be exposed via the API for the creation of transformation mappings. By adding this virtual relationship, they are able to be queried via `/api/clusters/:id?attributes=lans`

There will be a lans collection added to the API in a follow-up PR

app/models/ems_cluster.rb

@@ -34,6 +34,8 @@ class EmsCluster < ApplicationRecord
 
   virtual_has_many :storages,       :uses => {:hosts => :storages}
   virtual_has_many :resource_pools, :uses => :all_relationships
+  virtual_has_many :lans,           :uses => {:hosts => :lans}
+
   has_many :failover_hosts, -> { failover }, :class_name => "Host"
 
   include SerializedEmsRefObjMixin
@@ -230,6 +232,12 @@ def memory_reserve
     get_reserve(:memory_reserve)
   end
 
+  def lans
+    hosts = all_hosts
+    MiqPreloader.preload(hosts, :lans)
+    hosts.collect(&:lans).flatten.compact.uniq
+  end
+
   def effective_resource(resource)
     resource = resource.to_s
     unless %w(cpu vcpu memory).include?(resource)