Require the database cleanup only to remove USER and PASSWORD.

Due to limitations in django, we can't really remove or replace settings
dictionary once connection is instantiated. Therefore, all the code
that's playing with it will simply modify the existing dict.
As a result, removing the connection will only remove username &
password, not the whole entry.
Merge pull request #94 from mgorny/tests-fix

Require the database cleanup only to remove USER and PASSWORD.