rgw: don't log system requests in usage log

Fixes: 6889
System requets should not be logged in the usage log.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1427 from ceph/wip-6889

rgw: don't log system requests in usage log

Reviewed-by: Sage Weil <sage@inktank.com>