Add testnet configuration options to end-to-end tests

[marsella]

The config file options are currently hardcoded to use the Tezos sandbox (as well as self-delay, confirmation depth, and database locations), but we may want to run end-to-end tests with different config options.

This issue is done when the config options can be configured, maybe on the command line, to allow e2e tests to run on the Tezos testnet. The relevant config options are tezos_account and tezos_uri for both the merchant and customer, but the account might be hard-codeable - do testnet accounts ever run out of money?