Add configuration variables for pluggable payment processors
Configuration Pull Request
This PR adds configuration variables for configuring additional payment processors external to the ecommerce codebase.
These variables are required to utilize changes made by https://github.com/edx/ecommerce/pull/3297
External payment processors can be configured by setting the path to the payment processor class in `ECOMMERCE_ADDL_PAYMENT_PROCESSORS`, similar to how the [built-in processors are configured in ecommerce](https://github.com/edx/ecommerce/blob/2151b73b00bcb6528ae45945a460e2665ff0c5cd/ecommerce/settings/_oscar.py#L128).
Make sure that the following steps are done before merging:
[ ] A DevOps team member has approved the PR if it is code shared across multiple services and you don't own all of the services.
[ ] Are you adding any new default values that need to be overridden when this change goes live? If so:
[ ] Update the appropriate internal repo (be sure to update for all our environments)
[ ] If you are updating a secure value rather than an internal one, file a DEVOPS ticket with details.
[ ] Add an entry to the CHANGELOG.
[ ] If you are making a complicated change, have you performed the proper testing specified on the [Ops Ansible Testing Checklist](https://openedx.atlassian.net/wiki/display/EdxOps/Ops+Ansible+Testing+Checklist)? Adding a new variable does not require the full list (although testing on a sandbox is a great idea to ensure it links with your downstream code changes).
[ ] Think about how this change will affect Open edX operators. Have you updated the wiki page for the next Open edX release?