MongoDB hasn't been supported since Pike, it's time to remove the
deployment files. Starting in Stein, it's not possible to deploy MongoDB
anymore. It already changes the default zaqar management_store to
sqlalchemy and the zaqar messaging_store to redis, which is already
set by TripleO Heat Templates.
Change-Id: I470a7e8c25293b2f2cb5420be124a8809481478a