Need a way to delete account-keys from snapcraft integration tests

https://github.com/snapcore/snapcraft/pull/726 adds an integration test that creates account-keys on the store. It's currently disabled for non-fake stores because it would only ever be able to register any given key once.

We need a way to disable account-keys, and the integration test should really do both once we have that capability. However, we should perhaps also generate a fresh key for each integration test run and register that. If we do that, then we need a job that periodically clears out the test account-keys from our assertions database.