Provide explicit image create value for test_image_paginate case

Assign an explicit created_at datetime value to image db fixtures,
it be used to fixes race condition in DB driver test_image_paginate
case.

Closes-Bug: #1282400
Change-Id: I359633af4bb25cf1866b42d6dad3df658995b7bd
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Get rid of TaskDetails in favor of TaskStub

Change I0b406e4662cfd3cb496b71be77ad10a90c178baa introduces a new
class TaskStub. This class is designed to be a subset of what a
task is in the domain layer: it doesn't contain input, message and
result. TaskStub should be used when listing tasks in order to avoid
extra-overhead of transporting these text fields.
As a result, the TaskDetails introduced by change
I0b406e4662cfd3cb496b71be77ad10a90c178baa doesn't need to exist.

With this patch, the domain layer contains Task and TaskStub instead
of Task, TaskStub and TaskDetails.

partially implements bp async-glance-workers

Change-Id: I2a20b0c5033c6920749370355c2d672dec707c28
Merge "Provide explicit image create value for test_image_paginate case"
Replace assert* with more suitable asserts in unit tests

The following replacements were done in unit tests to have
clearer messages in case of failure:
- assertTrue(* is None) with assertIsNone
- assertTrue(* is not None) with assertIsNotNone
- assertTrue(* in *) with assertIn
- assertTrue(* not in *) with assertNotIn
- assertFalse(* in *) with assertNotIn
- assertTrue(* == *) with assertEqual
- assertTrue(* != *) with assertNotEqual

Change-Id: I0c47f991c3974e441335e71c9d26fab8a127f2ca
Merge "Get rid of TaskDetails in favor of TaskStub"
Merge "Replace assert* with more suitable asserts in unit tests"