Modify xbcloud to check for bucket existence and return 'ok' status when domain name can be resolved

bash-4.2$ xbstream -C /tmp -c sst_info | xbcloud --verbose put --storage=s3 --parallel=10 --md5 --s3-bucket=operator-testing some-name-2020-28-04-18:38
:33-full.sst_info

 About to connect() to operator-testing.minio-service port 9000 (#0)
*   Trying 172.30.159.107...
* Connected to operator-testing.minio-service (172.30.159.107) port 9000 (#0)
> HEAD / HTTP/1.1
Accept: */*
Accept-Encoding: gzip
Authorization: AWS4-HMAC-SHA256 Credential=some-access-key/20200428/us-east-1/s3/aws4_request, SignedHeaders=host;x-amz-content-sha256;x-amz-date, Sign
ature=a183832026e2beb00d120c62ac32d96f0811a1ceb1c544c10d3ad724ba7cfcd2
Host: operator-testing.minio-service:9000
X-Amz-Content-SHA256: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
X-Amz-Date: 20200428T193141Z
< HTTP/1.1 404 Not Found
< Content-Security-Policy: block-all-mixed-content
< Content-Type: text/plain; charset=utf-8
< Vary: Origin
< X-Amz-Request-Id: 160A12A12070A0E6
< X-Content-Type-Options: nosniff
< X-Xss-Protection: 1; mode=block
< Date: Tue, 28 Apr 2020 19:31:41 GMT
< Content-Length: 19
<
* Connection #0 to host operator-testing.minio-service left intact
200428 19:31:41 xbcloud: Successfully connected.