Braintree Blue: Default verification merchant id

If the merchant has configured the merchant_account_id and they want to
use the verify_card option, they probably want that verification run
against their configured merchant account, since Braintree Blue allows
for multiple merchant accounts. This sets a smart default for
verification_merchant_account_id, while still allowing it to be set
independently.

Closes #1244.
Braintree Blue: Kill expected to fail remote tests

They were really testing Braintree internal validations, which are out
of scope for the AM tests.