Add support for PartialTask list

- partial-task: Task that includes all the values except the 'Text'
columns(input, result & message).
- This patch is to have the database to return a partial-task list on users
invoking 'GET /v2/tasks'.
- Returning list of partial tasks will help in reducing the response time when the
user queries for the list of tasks.
- This patch also includes changes for making implementation and return values of
display task detail (GET /v2/tasks/{task_id}) and list tasks (GET /v2/tasks)
more explicit.

partially implements bp async-glance-workers

Change-Id: I4fbadc9a97e3147128c7c733384c7bb50918806f
Merge "Add support for PartialTask list"