Update Zaqar backend requirements

MongoDB is not Zaqar's default storage backend.
In fact, Zaqar don't have a default storage backend.
Considering this, pymongo shouldn't be in requirements.txt.
We still need it for the tests environments though.

Documentation about this situation will be added in the developers
guide and in the users guide in a follow up patch.

Change-Id: Ia9e039c671274fa32d90d8c31be6dca4d47bcbaf
Merge "Update Zaqar backend requirements"