added audit logging to credentials.
This backport is a bit different than the original patch,
since we don't have the adds caching of credentials
patch find on commit 479a2a0afa
and we were not able to backport it.
Also, since there was no flask support on stable/queens we needed
to backport the audit initiator on keystone/api/credentials.py
stable/rocky: https://review.opendev.org/#/c/711547
stable/stein: https://review.opendev.org/#/c/711545
master: https://review.opendev.org/#/c/664618
Change-Id: I028a86f44e049bcc7c54e844bfc91aa0b11cd541
(cherry picked from commit abf5cb6a55)