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 agree, but right now it is required because the current version we are using does not have upsert support with set_on_insert. We have to use underlying pymongo collection and update for this. This will be required for models which require upsert with set_on_insert queries for now (so I am guessing only TaskStatus). If we decide to carry and use the latest mongoengine version, then this can be removed.