add ceph bucket usage by user

[yanganto]

Propose command:

s3rs > usage {username}

Expect output:

{
  "bucket01": {
    "rgw.main": {
      "num_objects": 3,
      "size": 33,
      "size_actual": 12288,
      "size_kb": 1,
      "size_kb_actual": 12,
      "size_kb_utilized": 1,
      "size_utilized": 33
    }
  },
  "bucket01": {
    "rgw.main": {
      "num_objects": 300,
      "size": 444,
      "size_actual": 999,
      "size_kb": 100,
      "size_kb_actual": 100,
      "size_kb_utilized": 100,
      "size_utilized": 200
    }
  }
}

[yanganto]

Note:
usage s3://bucket & usage bucket show the usage of a bucket
user usage should use another command