[Spec] Move plugins spec to implemented

Change-Id: I729d7feff5ea0e0013a08c83293a5f2cecad9a13
Implements: blueprint split-plugins

doc/specs/implemented/split_plugins.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
+
+
+====================
+ Re-organize Plugins
+====================
+
+Move all plugins under rally/plugins to simplify Rally code base
+
+
+Problem description
+===================
+
+Rally code is coupled with Rally engine and infra as well as Openstack specific
+code. This makes contribution harder as new-comers need to understand Rally
+code as well as many different plugins. It also makes reviewing much harder.
+
+Proposed change
+===============
+
+Moving all plugins under a single directory, with "Openstack" as its
+sub-directory would make everything simpler.
+
+Alternatives
+------------
+
+None comes to mind.
+
+Implementation
+==============
+
+
+.. code-block:: shell
+
+  rally/
+   |
+   +-- plugins/
+           |
+           +-- common/
+           |     |
+           |     +-- runners/
+           |     +-- sla/
+           |     +-- contexts/
+           |     +-- scenarios/
+           |
+           +-- openstack/
+                 |
+                 +-- runners/
+                 +-- sla/
+                 +-- contexts/
+                 +-- scenarios/
+
+
+NOTE: looking at the current code base we can see that:
+
+#. All ``runners`` and ``sla`` will go under ``common``.
+#. All ``contexts`` will go under ``openstack``.
+#. Most of ``scenarios`` (except for ``dummy``) will go under ``openstack``.
+
+Assignee(s)
+-----------
+
+  - yfried
+
+  - boris-42
+
+Work Items
+----------
+
+- Move all OpenStack related plugins and code under ``plugins/openstack/`` and
+  all other plugins code under ``plugins/common/``.
+
+
+Dependencies
+============
+
+- Plugin unification