Remove common context usage from db model_query()

Currently, function model_query() from oslo.db.sqlalchemy.utils uses
is_use_context() function in order to determine whether a normal or an
admin user request is being processed. But usage of RequestContext is
not unified across OpenStack projects, so there is no way for oslo.db to
guarantee it processes project_id/deleted filters correctly here.

To remove this ambiguity, project_id/deleted filters should be passed
explicitly instead. At the same time, developers of OpenStack projects
are encouraged to provide wrappers for oslo.db model_query() function,
so that they could determine the values for project_id/deleted filters
depending on how they use RequestContext in their project.

Closes-bug: #1288846
Change-Id: Iec5cbbc44cf9626f0464ab684dff1ba37e006151
Merge "Remove common context usage from db model_query()"