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 should also be deleted. The fields_to_set_on_insert is used in exactly two places (tasks.py). Those two places should be rewritten so that the insert/upsert race condition is correctly handled there. Once fields_to_set_on_insert is no longer used then the vanilla save can be used which is what we want.