rgw: Replace deprecated Barbican payload endpoint with alternative

rgw: Replace deprecated Barbican payload endpoint with alternative

`GET /v1/secrets/{uuid}` with Header `Accept:
application/octet-stream` has been deprecated in favor of `GET
/v1/secrets/{uuid}/payload` in OpenStack Barbican.

The old API is still functional, but generates a warning level log
message on Barbican for every access.

Deprecation commit from 2015:
https://github.com/openstack/barbican/commit/b4b64be6f259acf4d8677eadbc35d37fab3a6df0

Signed-off-by: Marcel Lauhoff <marcel.lauhoff@clyso.com>
On-behalf-of: SAP marcel.lauhoff@sap.com