Fallback to None on missing Glance image attrs

It's possible for glance images to be missing attributes like name and
checksum. We'll just set those to None by default to avoid missing key
exceptions being raised.

Closes-Bug: #1308058
Change-Id: I85c42f8351763da201021a22f5ff0ebd62c6b2db
(cherry picked from commit 993facb3bf753d23332236dc05b397850bf99144)
Merge "Fallback to None on missing Glance image attrs" into stable/icehouse