Image caching: update image caching to use objects

Commit 1023e703bd41c2a42b1159af0d9e907e94440b34 added support
for objects. This patch ensures that the parsing of the instances
is correct.

The patch does the following:
1. In the method _list_running_instances it treats all instances
   as an object
2. Fixes the bug mentioned below. The code was making a string from
   None. This would lead to invalid image details.
3. Ensures that the unit tests actually pass instance objects instead
   of an instance dict
4. In relevant tests updates assertEquals to have the expected parameter
   first

Closes-bug: #1354664

Change-Id: Ib028baab4d4c823f22c371a83f7a813c24d77570
Merge "Image caching: update image caching to use objects"