Cleanup 'scheduled_at' from instances table

The 'scheduled_at' field is a part of the Nova instances.
Currently, the 'scheduled_at' field is no longer updated
by the Nova scheduler, instead the 'launched_at' field is
updated for the same purpose. The 'scheduled_at' column is
now redundant and should be removed as a part of DB clean up.

Change-Id: I7ccf1d225c983b69c05f2415094071bdafd63b85
Blueprint: cleanup-scheduled-at

specs/liberty/approved/cleanup-scheduled-at.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
+
+=====================================================
+Cleanup 'scheduled_at' column in nova instances table
+=====================================================
+
+https://blueprints.launchpad.net/nova/+spec/cleanup-scheduled-at
+
+The 'scheduled_at' field is a part of the Nova instances table
+Currently, the 'scheduled_at' field is no longer updated by the
+Nova scheduler, instead the 'launched_at' field is used.
+The 'scheduled_at' column is now redundant and should
+be removed as a part of DB clean up.
+
+Problem description
+===================
+
+The 'scheduled_at' column in the nova instances table is now
+redundant. This field was erstwhile used by the scheduler to denote
+the time of the instance scheduling. The commit that removed it is:
+https://review.openstack.org/#/c/143725/
+
+Now since all of the basic VM lifecyle operations go through the
+scheduler, the 'launched_at' field is sufficient for the above
+mentioned scenario.
+
+This is a nova DB cleanup effort to get removed the 'scheduled_at'
+column from the instances table and ensure backward compatibility.
+
+Use Cases
+----------
+
+This is a cleanup effort on the instances table.
+
+
+Project Priority
+-----------------
+
+This is a refactoring effort to make nova code cleaner.
+
+
+Proposed change
+===============
+Estimated changes are going to be in the following places:
+* Remove the sqlalchemy initializations for the 'scheduled_at' field
+
+* Remove all unit tests, involving this field.
+
+* Ensure that the NovaObjects adapt to this change for handling
+  migration related issues.
+
+
+Alternatives
+------------
+Continue to live with dormant code in nova.
+
+
+Data model impact
+-----------------
+The 'scheduled_at' column can be dropped from the Instances table since
+there appears to be no code, that updates it currently.
+
+
+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:
+
+* Sudipta Biswas sbiswas7
+
+
+Work Items
+----------
+
+1. Drop the column for 'scheduled_at' from Sqlalchemy model.
+2. Add migration to drop the column from the instances table.
+3. Change the instances object module for migration related issues.
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+The changes will be exercised through the existing CI.
+
+Documentation Impact
+====================
+
+None
+
+
+References
+==========
+