Comment 2 for bug 1790701

Looks like this was the regression:

https://review.openstack.org/#/c/541435/

Because before that the placement_context_manager was configured in the sqlalchemy DB API code. Now it's only configured in a few select places, one of which is not the online_data_migrations code.

This is also noticeable in devstack:

http://logs.openstack.org/08/599208/2/check/tempest-full/608d60a/controller/logs/devstacklog.txt.gz#_2018-09-02_15_04_31_949

But because we try/except Exception in online_data_migrations we ignored the failure.