Catch missing Glance image attrs with None

Glance is known to drop values from its response if they don't resolve to true.
 Previously in I67b7dd16a94fe60d873c012f6bd246ab24500d5a the None catch was
 removed as this WILL cause a call to glance for missing value and
 optimizations where made to attempt to pre-cache values that can be missing
 in expected ways. However there are still situations that can't be optimized.

Notably this can occur with disk_format, container_format, name, deleted,
 and checksum.

Test adapted from I85c42f8351763da201021a22f5ff0ebd62c6b2db by Mike Perez

Closes-bug #1308058
Co-Author: Mike Perez <thingee@gmail.com>

Change-Id: Icddead6f746d4d3ba652858eaae425acce8c177d
Merge "Catch missing Glance image attrs with None"