Updated/added DB Models Unit Tests and Bug Fixes

Modified "test_create" test to "test_create_get":
* After record is created, added call to get to verify
 record was actually written to DB.

Added unit tests:
* test_update
* test_delete_invalid_id
* test_delete
* test_get_delete_batch

Fixed DB model (in cue/db/) related bugs:

* In delete (base.py)
** added raising exception if record is not found.
* In delete_batch (base.py)
** delete function signature requires dictionary to be passed
* In update (base.py)
** updating 'updated_at' field date/time
* In base.py
** renamed TenantMix class to ProjectMix
* In models.py
** Added inheritance from Cluster class to ProjectMixin class
** Removed project_id instance variable from Cluster class

Change-Id: I80397749b7772bb117fbcd525f3f66cc9e47fc18