ambiguous column 'checksum' error when querying image-list(v2).

- removed ambiguity by prefixing the column with models.Image
- based on the image_get_all() method logic, changed the
filters.get(checksum) to filters.pop(checksum) to
avoid multiple addition of same criteria in the where clause.
- modified the functional db test to include this failing scenario.

Fixes bug 1205330

Change-Id: Ic2b5b9bed7c02865d66e2f92846dcc9c0e7e2f97
Merge "ambiguous column 'checksum' error when querying image-list(v2)."