[Spec] Reorder plugins

Rally has billions of plugins and it becomes really hard for newbies
to understand how Rally code is organized and how it works.

So let's split framework and plugins

Example https://review.openstack.org/#/c/176092/

Change-Id: I6980c0dcbeeabf79e774c69cd27e5e4efca0fc50

doc/specs/in-progress/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