Fixes exposing trace during calling image create API

Now Glance will return trace instead of standard HTTP error
during create image with bad location and nonempty body.
The root cause is nonempty request body will make add image
as synchronous action instead of queuing it. And then, the
image upload will fail due to the bad location. As a result,
user will run into the exception trace instead of friendly
HTTP error.

Based on current design, it's invalid to provide multiple
image sources in one request. So this fix is adding an
explicit check to make sure there is only one image source.

Fixes bug: 1185055

Change-Id: I204aa14fa35e50aeb29b6dc22bbf5bbc66a2fb42
Merge "Fixes exposing trace during calling image create API"