Remove implicit RequestContext monkey patch

[Cedric Brandily]

oslo.db 1.12 decorates IMPLICITLY oslo_context RequestContext[1] with session management features when enginefacade module is imported.

We should not AUTOMATICALLY and IMPLICITLY decorate RequestContext as it makes debugging more context, some applications have their own session management in a RequestContext subclass[2][3] or don't want any

[1] https://github.com/openstack/oslo.db/blob/1.12.0/oslo_db/sqlalchemy/enginefacade.py#L782
[2] https://github.com/openstack/neutron/blob/master/neutron/context.py
[3] at least until oslo.db>1.12 is added in neutron requirements.txt

[Cedric Brandily (cbrandily)]

PS: implicit monkey patch has broken neutron: https://review.openstack.org/194844