Add rally job related files

* Add README.rst file with useful description
* Add extra and plugins directory with corresponding README files
* Add rally task designate.yaml that will be actually run in gates

Infra related patch is here:
https://review.openstack.org/#/c/135132/

Change-Id: Ida3c3aa24f63592f9c4bd7143d30377fddd021a6

rally-scenarios/README.rst

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+Rally job related files
+=======================
+
+This directory contains rally tasks and plugins that are run by OpenStack CI.
+
+Structure:
+
+* zaqar-zaqar.yaml is rally task that will be run in gates
+
+* plugins - directory where you can add rally plugins. Almost everything in
+  Rally is 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 path in rally tasks.
+  Files will be in ~/.rally/extra/*
+
+
+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-scenarios/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-scenarios/plugins/README.rst

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

rally-scenarios/zaqar-zaqar.yaml

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+---
+  ZaqarBasic.create_queue:
+    -
+      args:
+        name_length: 10
+      runner:
+        type: "rps"
+        times: 1000
+        rps: 50
+      sla:
+        max_failure_percent: 0