Add support for image size in v2 api upload

Some backend stores e.g. RBD, will fail if told to create an
image without a valid size (RBD will fail to write to a zero-size
image). Here we add support to allow the image size to be provided
when doing an upload. The result is that the upload content-length
will be set if available either from checking the supplied file
object or as provided by user.

Closes-Bug: 1220197
Change-Id: Ia1f2ea5680a139750d931591949b3e0058148b4b