Add unit/function tests for DB Mixin classes

[Davide Agnello]

Unit/functional test coverage for:
* LookupTableMixin
* IdMixin
* ProjectMixin
* SoftDeleteMixin

[Min Pae (sputnik13)]

[13:38:37] @vipul: davideagnello: just looking at this patch around the SoftDeleteMixin
[13:39:02] @vipul: davideagnello: you mentioned that we already have db api tests that exercise soft-delete? https://github.com/stackforge/cue/blob/master/cue/tests/db/test_api.py
[13:39:09] @vipul: i don't see any of those tests there
[13:39:45] @vipul: or here https://github.com/stackforge/cue/blob/master/cue/tests/db/test_models.py
[13:40:23] davideagnello: vipul: I said we have tests that create cluster objects from DB layer and objects layer not just API through post
[13:40:43] davideagnello: Soft delete is being tested indirectly through the API test that I have added in that patch
[13:40:49] davideagnello: through issuing a delete in the api
[13:41:01] davideagnello: which then makes use of softdelete functionality
[13:41:31] davideagnello: timestamps are being checked for relative time compared to created_at and updated_at
[13:41:36] @vipul: davideagnello: right.. that makes sense. It would be nice to have these things verified from the dbapi directly
[13:42:08] davideagnello: vipul: yes it would, I noticed that we don't have tests for any of our mixin db classes at all yet.
[13:43:05] davideagnello: I can add some tests directly at the db api level as part of this patch or can we add them as part of QA (will create work item to capture this coverage)