No more soft delete

There was widespread agreement at the YVR summit not to soft-delete
any more things. To codify this, we should remove the SoftDeleteMixin
from NovaBase.

blueprint no-more-soft-delete

Change-Id: If9200823f472fd3e9c32a8e61f235539ec7de8a5

specs/mitaka/approved/no-more-soft-delete.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+===================
+No more soft delete
+===================
+
+https://blueprints.launchpad.net/nova/+spec/no-more-soft-delete
+
+There was widespread agreement at the YVR summit not to soft-delete any more
+things. To codify this, we should remove the SoftDeleteMixin from NovaBase.
+
+Problem description
+===================
+
+Soft deletion of rows imposes a management overhead to later delete or archive
+those rows. It has also proved less necessary than initially imagined. We would
+prefer additional soft-deletes were not added and so it does not make sense to
+automatically inherit the `SoftDeleteMixin` when inheriting from NovaBase.
+
+Use Cases
+---------
+
+As an operator, adding new soft deleted things means I need to extend my
+manual cleanup to cover those things. If I don't, those tables will become
+very slow to query.
+
+As a developer, I don't want to tempt operators to read soft-deleted rows
+directly. That risks turning the DB schema into an unofficial API.
+
+As a developer/DBA, providing `deleted` and `deleted_at` columns on tables
+which are not soft-deleted is confusing. One might also say it's confusing to
+soft-delete from tables where deleted rows are never read.
+
+Proposed change
+===============
+
+This spec proposes removing the `SoftDeleteMixin` from NovaBase and re-adding
+it to all tables which currently inherit from NovaBase. The removal of
+SoftDeleteMixin from those tables which don't need it will be left for future
+work.
+
+Alternatives
+------------
+
+We could not do this. This means we need an extra two columns on new tables
+and it makes it slightly easier to start soft-deleting new tables.
+
+Data model impact
+-----------------
+
+None.
+
+REST API impact
+---------------
+
+None.
+
+Security impact
+---------------
+
+None.
+
+Notifications impact
+--------------------
+
+None.
+
+Other end user impact
+---------------------
+
+None.
+
+Performance Impact
+------------------
+
+None.
+
+Other deployer impact
+---------------------
+
+None.
+
+Developer impact
+----------------
+
+None.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  alexisl
+
+Other contributors:
+  None
+
+Work Items
+----------
+
+* Remove `SoftDeleteMixin` from NovaBase.
+* Add it to all models which inherited from NovaBase.
+
+Dependencies
+============
+
+None.
+
+Testing
+=======
+
+None.
+
+Documentation Impact
+====================
+
+None.
+
+References
+==========
+
+None.
+
+History
+=======
+
+.. list-table:: Revisions
+   :header-rows: 1
+
+   * - Release Name
+     - Description
+   * - Liberty
+     - Introduced
+   * - Mitaka
+     - Simplified and re-proposed