Add vendor extensible framework

This proposal to add provision to plug-in vendor
drivers/podmanagers in valence.

Change-Id: Ie61871f319d8317842c5ceedb9b514f47817713a

specs/pike/approved/vendor_extensible_framework.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
+
+================================
+Add Vendor Extensible framework
+================================
+
+https://blueprints.launchpad.net/openstack-valence/+spec/add-vendor-extensible-framework
+
+This spec is to provide vendor extensible framework in valence to support
+management of pooled infrastructure using drivers other than
+redfish.
+
+Problem description
+===================
+
+Currently, valence supports only redfish driver, which limits it's use case
+to specific hardware's which support redfish only. To provide extensibility
+for other non-Redfish disaggregated/composable hardwares to integrate with
+valence, framework needs to be in place where other vendors will be able to
+plug driver's specific to manage their pooled infrastructure.
+
+This blueprint still proposes redfish as the default driver to manage
+resources, but it will provide an abstract layer to plug vendor drivers inside
+valence.
+
+Proposed change
+===============
+
+Note: The following proposed change assumes valence have multi-podm support,
+depends on https://blueprints.launchpad.net/openstack-valence/+spec/refactor-to-support-mutli-podmanager
+
+* New field 'type' to be added while creating pod manager. If none specified,
+  by default instantiates default podmanager, PodManagerBase as defined in
+  multi-podm patch.
+* 'type' would take following values: 'default', '2.1', 'lenovo' etc.
+* Use stevedore to load respective podmanager based on 'type' dynamically and
+  store the cache to avoid loading each time.
+* New module manager.py to be added, to load respective podmanager instance.
+* Controller will use generic interface, to call respective function on Manager
+  instance.
+* Create a new folder where all the podmanagers files would be placed and
+  move the current redfish folder to the same.
+
+API --> Controller --> Manager --> Podmanager
+
+The change will be more of extension of multi-podm support to make it more
+generic, no major changes would be required.
+
+Alternatives
+------------
+None
+
+Data model impact
+-----------------
+New field 'type' would be added to pod_manager DB.
+
+REST API impact
+---------------
+podmanager create request POST http://127.0.0.1/pod_managers would
+accept new attribute driver(optional field) to specify.
+
+Versioning impact
+-----------------
+
+
+Other end user impact
+---------------------
+
+
+Deployer impact
+---------------
+None
+
+Developer impact
+----------------
+None
+
+Valence GUI / Horizon impact
+----------------------------
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+ ramineni
+
+Other contributors:
+
+Work Items
+----------
+* Modify podmanager DB to add new field 'type'
+* Update controller to use generic interface exposed by Manager class
+  to invoke the methods
+* Add new folder and move the current redfish implementation to the same.
+
+Dependencies
+============
+Multi-podmanager support
+
+Testing
+=======
+
+Documentation Impact
+====================
+
+References
+==========
+multi-podm patch : https://review.openstack.org/#/c/445360/8