Fix nova image-show with queued image

Currently, the nova image-show command cannot be used with queued images,
because they don't have certain attributes set yet. Nova will raise
AttributeError, which causes a 500 to be returned from the API.

Change-Id: I7d73b891545492e3e247eebdf02507cb630eb250
Closes-bug: #1321186
(cherry picked from commit 4c5dea6a5103a141b0ca13dcd57b32b3261180d1)
Merge "Fix nova image-show with queued image" into stable/icehouse