Add bucket exists check before checking object exists while uploading image

[KeerthanaAP]

Added bucket exists check before verifying whether the object exists when cos-instance-name is provided.

./pvsadm image upload -b power1-oss-bucket -f karthik-centos-8.ova.gz -n New-keer-cos -r us-south
I0507 22:46:33.462260   60503 upload.go:134] Bucket power1-oss-bucket not found in the instance New-keer-cos provided
I0507 22:46:35.949740   60503 upload.go:228] Creating a new bucket power1-oss-bucket
I0507 22:46:40.552761   60503 s3client.go:209] Waiting for bucket power1-oss-bucket to be created.
I0507 22:46:40.962257   60503 s3client.go:269] uploading the file karthik-centos-8.ova.gz

[mkumatag]

/lgtm

[ppc64le-cloud-bot]

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This pull-request has been approved by: KeerthanaAP, mkumatag

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