Fix paypal payment processor default configuration
Fix paypal payment processor default configuration

The `cancel_url` configuration setting is not used by the paypal payment
processor in ecommerce. Instead, the payment processor uses the
`cancel_checkout_path` configuration parameter.

This error has been reported by two different users:
https://discuss.overhang.io/t/ecommerce-plugin-500-error-on-fresh-setup
https://openedx.slack.com/archives/C02SNA1U4/p1573488822105900?thread_ts=1573488555.104900&cid=C02SNA1U4

The proposed fix has worked in both cases.

edx/ecommerce,eduNEXT/edunext-ecommerce,edx/ecommerce,edx/ecommerce,edx/ecommerce,eduNEXT/edunext-ecommerce,eduNEXT/edunext-ecommerce,eduNEXT/edunext-ecommerce