Add a spec for Pluggable do actions

Change-Id: Ia9c07215609c72671ce53e6a1e8e8390f3207c23
blueprint: pluggable-do-actions.rst

specs/pluggable-do-actions.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
+
+=================================
+Pluggable architecture for bareon
+=================================
+
+https://blueprints.launchpad.net/bareon/+spec/pluggable-do-actions
+
+At the current state bareon is monolitic package.
+If one wants to add new drivers or do actions, then those changes should be
+landed into bareon's repo first. There's no convenient way to develop
+something as a 3rd party component or re-use actual codebase as a framework.
+That's why we need to introduce pluggable architecture.
+
+Problem description
+===================
+
+Currenly, there're few flaws in current bareon architecture:
+  - It's purely monolitic package. It can't be split to few packages like
+  bareon-base, bareon-provisioning, bareon-image-building, bareon-partiioning.
+  For example, we'll never build in bootstrap loaded node, but having bareon
+  package installed into it. It bring additional package dependencies which are
+  needed only for image building, but obviously not needed for provisioning.
+  - Like it was said above, there's no convenient way to develop out of core.
+  This is the main stopper for external contibutors.
+  - It's hard to introduce new functionality or re-use existing one.
+  - It glues few do_actions into single combo and prevents from running actions
+  separately. For example, run partitioning without provisioning.
+
+Therefore, pluggable architecture is really necessary.
+
+Proposed change
+===============
+
+Data drivers which prepare objects for manager should re-introduced as
+pluggable extensions. The same applies to manager's do actions.
+Therefore new directory for actions will be created:
+
+  bareon/actions/
+
+Drivers will be stored under already existent directory:
+
+  bareon/drivers
+
+Manager will be improved in order to work with this pluggable actions and
+driver. At least, manager should know how to check the presence of pluggable
+extensions and validate that input data already contains all necessary
+information for being be processed later.
+
+stevedore will be used as a convenient extension manager.
+
+Alternatives
+------------
+
+There's no alternative way to introduce pluggable extensions.
+Besides, keeping all things as is not the right way to go with.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  `Alexander Gordeev`_
+
+Mandatory Design Reviewers:
+  `Evgeny Li`_
+  `Vladimir Kozhukalov`_
+
+Milestones
+----------
+
+Target Milestone for completion:
+  1.0.0 
+
+Work Items
+----------
+
+- Introduce pluggable extension.
+- Rework existing drivers/actions according to the new architecture
+
+Dependencies
+============
+
+Doesn't require new dependencies as stevedore has been already included into
+dependencies.
+
+----------
+References
+----------
+
+.. _`Alexander Gordeev`: https://launchpad.net/~a-gordeev
+.. _`Vladimir Kozhukalov`: https://launchpad.net/~kozhukalov
+.. _`Evgeny Li`: https://launchpad.net/~rustyrobot