Size should be an optional argument for SOURCE-based images of OS type
When adding an image of SOURCE type, the size needs to be passed as a mandatory argument.
But sometimes a SOURCE image lives on a remote host (http) so the user does not know
the exact size beforehand.
In previous versions, the behavior for the "fs" datastore was to download the image to from the remote location. The "cp" datastore script does this, I believe, and returns the image size as an output.
The size argument should therefore be optional (at least in some cases), as it will be provided by datastore drivers which adjust to the specification.
Tested in 5.1.80