keep common requirements in global config about gringotts HOT 2 OPEN

Though a default currency is required by all gateways, IMO it is unlikely that it will be same across all gateways:

• Large multi-national merchants might use multiple gateways because some gateways offer better/cheaper service in some regions, but not in others. In that case, each of these gateways would be configured for a different currency.
• All gateways should set a default currency (since this is truly very specific to a gateway and not merchant dependent), but where? We could do something like this or add it in the required_config for all gateways.

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After the introduction of #62 and merges in #71 #85 it becomes evident that the lib users can use money lib to create money object.

Responsibility of creating a correct money struct lies on the developer and follows the rules and constraints of money ex_money lib.

Hence, having default_currency in the global config does not make much sense now.

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