Mongoengine taskstatus

This contains -

1. Schema definition for TaskStatus inherited from mongoengine.Document
2. Added fields.py to define custom fields used in pulp that are not available from mongoengine.
3. Updated save method on TaskStatus to do the validation of fields as well as upserts.
4. Updated task status manager with mongoengine queries
5. Added and updated unit tests in various files as appropriate.
6. Enabled a bunch of unit tests in test_dispatch.py which were not running because of wrong test names.

Since there are a lot of in-place changes, pull request diff looks confusing at some places. It is better to look at the whole file instead of the diff.

**Note**: This PR contains changes strictly related to conversion to mongoengine. Getting rid of TaskStatusManager and moving all it's functionality to the TaskStatus model plus updating a ton of relevant unit tests is being handled in a separate branch remove-taskstatus-manager.
@skarmark I think the set_task_* code is called from exactly 1 place. It was used by the Reply class, but that can be deleted. All other calls to set_task_* are tests. I think these methods should go away, and the code from them should be moved directly to the server/pulp/server/agent/direct/services.py module. I can't think of a reason why updating the status needs to be done in multiple places.