Update stemcell:upload_os_image

Add requirement/ability to pass in the S3 key on the command line.
Remove Bosh::Stemcell:OsImageUploader as it is no longer necessary.

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>
Add ci:publish_os_image_in_vm rake task

Also includes a refactor of StemcellVm.

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>