Validate ChargebackRateDetail#chargeback_rate
Validate ChargebackRateDetail#chargeback_rate

In the API chargeback rate details can be created successfully with an
invalid chargeback_rate_id. Adding this validation ensures that this
can't happen at the model level.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1328940
(transferred from ManageIQ/manageiq@6241a424a35cd03d26d75ca0c049314f7c5e4ee5)

ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic