Do not use BigDecimal in euwe

Euwe uses ruby 2.3.1 which does not coalesce floats and BigDecimals
well.

Try for yourself

    BigDecimal.new(1) <= Float::INFINITY

or

    Float::INFINITY >= BigDecimal.new(1)

Fixes ManageIQ#13591.
Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1416001

app/models/chargeback/consumption_without_rollups.rb

@@ -50,7 +50,7 @@ def current_value(metric)
                          when 'derived_memory_available'
                            resource.hardware.try(:memory_mb)
                          when 'derived_vm_allocated_disk_storage'
-                           resource.allocated_disk_storage
+                           resource.allocated_disk_storage.try(:to_f)
                          end
       @value[metric]
     end