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:
openstack/barbican@b4b64be

Signed-off-by: Marcel Lauhoff <[email protected]>
On-behalf-of: SAP [email protected]

Author: irq0

src/rgw/rgw_kms.cc

@@ -929,6 +929,7 @@ static int request_key_from_barbican(const DoutPrefixProvider *dpp,
   }
   concat_url(secret_url, "/v1/secrets/");
   concat_url(secret_url, std::string(key_id));
+  concat_url(secret_url, "/payload");
 
   bufferlist secret_bl;
   RGWHTTPTransceiver secret_req(cct, "GET", secret_url, &secret_bl);