mgr/dashboard: Audit REST API calls

mgr/dashboard: Audit REST API calls

Fixes: https://tracker.ceph.com/issues/36193
Enable API auditing with 'ceph dashboard set-audit-api-enabled true' (default is false). If you do not want to log the request payload, then disable it via 'set-audit-api-log-payload false' (default is true).

Example output:
2018-10-08 10:25:21.850994 mgr.x [INF] [DASHBOARD] from='https://[::1]:44410' path='/api/auth' method='POST' user='None' params='{"username": "admin", "password": "***", "stay_signed_in": false}'

Signed-off-by: Volker Theile <vtheile@suse.com>