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. 
I think this method should be deleted and anything that uses it should get the collection directly. Also PulpCollection should not be used by the new mongoengine objects. The only thing I think it provides to is auto-retry support so we need to think of how to accommodate that without using PulpCollection. This will require some discussion and I don't have an answer without more thought.