CVE verification: Ensure last_override_config does not leak sensitive info

[bmbouter]

This Pulp CVE describes issue 3521 whereby the previous run's override config can be read as the attribute last_override_config on both importers and distributors. The decision (on the issue) was made to remove this attribute altogether due to it's security implications.

Without the fix, when you set an override_config, you'll be able to read that data when viewing the importer or distributor again. With this fix that data will always show up as 'last_override_config': {}.

This should be verified for both Importers and Distributors.

Details on how I tested this during development (with commands) is here: https://pulp.plan.io/issues/3521#note-15

This is a backwards compatible change for 2.16.2 and a release note was added for it.