Fix missing disk resources on API status.

If the disk resource type doesn't exist in OAR's database, we add it to
the possible types so we can have it (empty) im the returned JSON.
Some applications need the disks key, even empty.

app/controllers/sites_controller.rb

@@ -48,7 +48,10 @@ class SitesController < ResourcesController
       ]
     }
 
+    # Select the possible resources type from database. In disk case, even if
+    # it doesn't exists we want it in the result (as an empty Hash value).
     rtypes = OAR::Resource.select(:type).distinct.map{ |t| t.type }
+    rtypes.push('disk') unless rtypes.include?('disk')
     expected_rtypes = []
 
     rtypes.each do |oar_type|