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
Merge "Fallback to None on missing Glance image attrs"