Add support for URL based volumes.

If the API looks ok, I will add it to the other API files.

thanks
--brendan
These API entities aren't factored correctly. Local disk and PD are data targets, not sources. 

Sources could be as-is, empty, or populated by command, by image, or by URL, assuming the latter could be made with auth. 

It's also the case that the API representation is not extensible at all. :-(

/cc @thockin