Add an association for Datacenters

Datacenters are a subclass of EmsFolder but have their own inventory
collection.  This will allow us use the standard association attribute
for an inventory_collection rather than having to define a custom arel
like Ovirt does today.

app/models/ext_management_system.rb

@@ -61,6 +61,7 @@ def self.api_allowed_attributes
   has_many :blacklisted_events, :foreign_key => "ems_id", :dependent => :destroy, :inverse_of => :ext_management_system
   has_many :miq_alert_statuses, :foreign_key => "ems_id", :dependent => :destroy
   has_many :ems_folders,    :foreign_key => "ems_id", :dependent => :destroy, :inverse_of => :ext_management_system
+  has_many :datacenters,    :foreign_key => "ems_id", :class_name => "Datacenter", :inverse_of => :ext_management_system
   has_many :ems_clusters,   :foreign_key => "ems_id", :dependent => :destroy, :inverse_of => :ext_management_system
   has_many :resource_pools, :foreign_key => "ems_id", :dependent => :destroy, :inverse_of => :ext_management_system
   has_many :customization_specs, :foreign_key => "ems_id", :dependent => :destroy, :inverse_of => :ext_management_system