CyberSource API Update

Add verification for CyberSource2 implementation.
Clean up shopping cart processor API.
Fix UUID JSON serialization bug in CyberSource2 implementation.
Update test suite to use new CyberSource2 implementation.
Fix i18n messages in CyberSource2
Enable CyberSource2 implementation by default.

I've changed the Django settings to allow for multiple payment configurations.  Before, we were retrieving the first element in the `CC_PROCESSOR` dict, which won't work for multiple configurations (since dictionary keys don't have a defined order).  I've added a new setting to indicate which processor to use:
```
CC_PROCESSOR_NAME = "CyberSource2"
CC_PROCESSOR = {
    "CyberSource": {
        # ...
    },
    "CyberSource2": {
        # ...
    }
}
```

@dianakhuang please review the code changes.

@mattdrayer @feanil I've made some changes to the Django settings for payment processing, which may require changing configuration.  Please review.

JIRA issues:
[ECOM-107](https://openedx.atlassian.net/browse/ECOM-107)
[ECOM-108](https://openedx.atlassian.net/browse/ECOM-108)