Fix deletion of Default Container Image Rate

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1486658

Fix deletion of Default Container Image Rate in Cloud Intel -> Chargeback
-> Rates so that the Rate can't be deleted and error message is displayed.

app/controllers/chargeback_controller.rb

@@ -216,7 +216,7 @@ def cb_rates_delete
         self.x_node = "xx-#{cb_rate.rate_type}"
       end
     end
-    process_cb_rates(rates, 'destroy') unless rates.empty?
+    process_cb_rates(rates, 'destroy') if !rates.empty? && cb_rates_default(rates)
     if flash_errors?
       javascript_flash
     else
@@ -226,6 +226,18 @@ def cb_rates_delete
     end
   end
 
+  # Check for Default Rates which shouldn't be deleted
+  def cb_rates_default(rates)
+    rates.each do |rate_id|
+      cb_rate = ChargebackRate.find(rate_id)
+      if cb_rate.default? || cb_rate.description == 'Default Container Image Rate'
+        add_flash(_("Default Chargeback Rate cannot be removed."), :error)
+        return false
+      end
+    end
+    true
+  end
+
   # AJAX driven routine to check for changes in ANY field on the form
   def cb_assign_field_changed
     return unless load_edit("cbassign_edit__#{x_node}", "replace_cell__chargeback")