Update rally-jobs files

*) Rename rally-scenarios that is quite misleading to rally-jobs.
   rally-jobs makes much more sense, cause it actually contains files
   related to rally job

*) Update rally-jobs/README.rst to add more info

*) Update rally-jobs/plugins/README.rst to expaling plugins

*) Add new directory rally-jobs/extra, this directory is copy pasted
   in gates and can be used for files that are required by some of
   benchmarks

Change-Id: I6d0c0435a4bb4658ddf4adb871bc36ab8c157f3e

rally-jobs/README.rst

@@ -0,0 +1,29 @@
+Rally job related files
+=======================
+
+This directory contains rally tasks and plugins that are run by OpenStack CI.
+
+Structure
+---------
+
+* plugins - directory where you can add rally plugins. Almost everything in
+  Rally is a plugin. Benchmark context, Benchmark scenario, SLA checks, Generic
+  cleanup resources, ....
+
+* extra - all files from this directory will be copy pasted to gates, so you
+  are able to use absolute paths in rally tasks.
+  Files will be located in ~/.rally/extra/*
+
+* neutron-neutron.yaml is a task that is run in gates against OpenStack with
+  Neutron Service deployed by DevStack
+
+Useful links
+------------
+
+* More about Rally: https://rally.readthedocs.org/en/latest/
+
+* How to add rally-gates: https://rally.readthedocs.org/en/latest/rally_gatejob.html
+
+* About plugins:  https://rally.readthedocs.org/en/latest/plugins.html
+
+* Plugin samples: https://github.com/stackforge/rally/tree/master/doc/samples/plugins

rally-jobs/extra/README.rst

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+Extra files
+===========
+
+All files from this directory will be copy pasted to gates, so you are able to
+use absolute path in rally tasks. Files will be in ~/.rally/extra/*
+

rally-jobs/plugins/README.rst

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+Rally plugins
+=============
+
+All *.py modules from this directory will be auto-loaded by Rally and all
+plugins will be discoverable. There is no need of any extra configuration
+and there is no difference between writing them here and in rally code base.
+
+Note that it is better to push all interesting and useful benchmarks to Rally
+code base, this simplifies administration for Operators.