KAZOO-2881: System config to disable prorate of braintree subs

I think this key should reflect the conditional natute of the flag; maybe "should_prorate" would convey the intent better.

I also think this should be a function in braintree_subscription instead of coded here:
```
case braintree_subscription:should_prorate(Subscription) of
    'true' -> update_options();
    'false' -> P
end
```