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. 
Totally agreed, and there is a way to get rid of it completely. Just not in the version that we are using. Because of rhel6, we are using mongoengine's version which is more than 2 years old. Upsert's set_on_insert support is the only strong reason I have been able to find to move to the newer version though, so not sure whether it is worth carrying it ourselves.