Fixed issue when bucket exists, but 'usage' is empty due RGW bug

[k0ste]

Traceback (most recent call last):
  File "./check_ceph_rgw_api", line 116, in <module>
    sys.exit(main())
  File "./check_ceph_rgw_api", line 95, in main
    bucket_usage_kb = usage_dict['rgw.main']['size_kb_actual']
KeyError: 'rgw.main'

This possible with old RGW's due multipart bugs, for example
this bucket impossible to delete, because multiparts objects
is not exits. So better just skip this bucket.

{
    "object": "_multipart_obj_tag.txt.2~KFWA-rRXHsFLc45G3-rDFimIM4yD1aU.meta"
}
{
    "existing_header": {
        "usage": {
            "rgw.multimeta": {
                "size": 0,
                "size_actual": 0,
                "size_utilized": 0,
                "size_kb": 0,
                "size_kb_actual": 0,
                "size_kb_utilized": 0,
                "num_objects": 1
            }
        }
    },
    "calculated_header": {
        "usage": {
            "rgw.multimeta": {
                "size": 0,
                "size_actual": 0,
                "size_utilized": 0,
                "size_kb": 0,
                "size_kb_actual": 0,
                "size_kb_utilized": 0,
                "num_objects": 1
            }
        }
    }
}

[k0ste]

@blemmenes I was catch this bug on my cluster, resolve #15

[blemmenes]

@k0ste , LGTM, no issues on our clusters. I'll merge.