Don't log the credentials by default

Even in case of DEBUG level logging credentials (especially those that
give admin level access) should not be saved into log files.

This way of handling it has the side effect that if someone uses
password "password", it will be replaced in another place too... but
password "password" or some other keyword that can be found in the
request itself was a pretty bad idea to begin with.

Shell utilities are not affected and the verbose mode will still
display the passwords to make debugging easy.

Implements: blueprint limit-credentials-logging
Change-Id: I50d0ebbfbd44c7a5b162d9334b4fdbda67e5c28d