Add replace cold migration to use nova API spec

This patch adds replace-cold-migrate-to-use-nova-migration-api spec.

Implements: blueprint replace-cold-migrate-to-use-nova-migration-api
Change-Id: Ib83d391fe2cb2ba63a1ce20ec39011dbb0755a35

specs/rocky/approved/replace-cold-migrate-to-use-nova-migration-api.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
+
+================================================
+Replace cold migration to use Nova migration API
+================================================
+
+https://blueprints.launchpad.net/watcher/+spec/replace-cold-migrate-to-use-nova-migration-api
+
+
+Problem description
+===================
+
+Currently watcher implements instance cold migration by it's own for
+specifying destination node.
+The implementation includes instance creation. Since administrator runs
+watcher API, instance migrates to admin project.
+
+Use Cases
+----------
+
+Migrate action executes instance cold migration if migration_type is cold.
+
+
+Proposed change
+===============
+
+Since v2.56, Nova `migrate Server(migrate Action)`_ API has host option.
+By replacing watcher_non_live_migrate_instance method in common/nova_helper.py
+to use the API, we can simply solve the problem.
+
+Alternatives
+------------
+
+It may be the alternative to create instance by temporary user in the
+project where instance is in.
+But this solution remains quota problem. During temporary user lives and
+reached quota limit, new user can not be created.
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+We may expect cold migration becomes faster because current implementation
+calls many Nova and Neutron API.
+
+
+Other deployer impact
+---------------------
+
+* Administrator can see migration by watcher using Nova List Migrations API.
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  <nakamura-h>
+
+Other contributors:
+  <None>
+
+Work Items
+----------
+
+* Replace watcher_non_live_migrate_instance method
+  in common/nova_helper.py to use Nova migrate Server(migrate Action) API.
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+We should expect current test code can test this replacement.
+
+
+Documentation Impact
+====================
+
+None
+
+
+References
+==========
+
+.. _migrate Server(migrate Action): https://developer.openstack.org/api-ref/compute/#migrate-server-migrate-action