Add support of cinder's new attachment API

Add spec for cinder's new attachment API support and multiattach handling.

Change-Id: Ic5247dd0abf65d93fb1119efd9db06923cc2539e

specs/xena/approved/glance_store/attachment-api-and-multiattach-support

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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
+
+=======================================================
+Add new cinder's attachment API and multiattach support
+=======================================================
+
+https://blueprints.launchpad.net/glance-store/+spec/attachment-api-and-multiattach-support
+
+Make glance cinder store use cinder's new attachment APIs for image operations.
+
+Problem description
+===================
+
+Currently when we make simultaneous requests to glance for using the image to
+create multiple volumes or launch multiple instances, the same image volume needs
+to be attached multiple times to serve the requests and since the support doesn't
+exist in glance cinder store, some of the requests fails.
+
+Proposed change
+===============
+
+Cinder's new attachment API was introduced in microversion 3.27 which should be
+supported by cinder and is required to pass while making calls through cinderclient.
+There were 2 new methods introduced, attachment_complete in MV 3.44 and passing
+attach mode during attachment_create in MV 3.54 which are also required and hence
+we will be using MV 3.54 for attachment related operations.
+All these changes are required prior to adding the multiattach handling.
+
+This should be a 2 stage change:
+
+1) Replace existing API calls to cinder for attachment with new attachment API
+calls
+
+2) Add multiattach handling for glance cinder usecase
+
+Alternatives
+------------
+
+Use cinder's image volume cache that will clone the first bootable volume created
+from image and subsequent requests will result in cinder's clone operation instead
+of downloading the image from glance.
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+The current number of calls to cinder will be reduced resulting in performance
+improvement. Example: initialize_connection and attach calls will be replaced
+by attachment_update and similarly terminate_connection and detach calls will
+be replaced by attachment_delete.
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+* whoami-rajat
+
+Other contributors:
+  None
+
+Reviewers
+---------
+
+Core reviewer(s):
+
+* abhishek-kekane
+* jokke
+* rosmaita
+* smcginnis
+
+Work Items
+----------
+
+* Replace existing calls to old attachment methods with the new attachment API
+calls
+* Modify cinderclient calls to include microversion
+* Add code for multiattach handling
+* Add appropriate unit tests
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+Unit tests to have the code coverage and the new attachment code path will
+be tested with the glance cinder tempest job running on glance gate.
+
+Documentation Impact
+====================
+
+None
+
+References
+==========
+
+None