[spec] Add performance degradation SLA plugin

This is document describes Rally SLA performance
degradation plugin.
This is plugin needed for reliability and density testing
of OpenStack clusters.

More info about testing available in openstack/performance-docs

Change-Id: Ic23d4e21e5bacbc949990364956edf06ac7e99f7

doc/specs/in-progress/sla_pd_plugin.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
+
+==================================
+SLA Performance degradation plugin
+==================================
+
+Problem description
+===================
+
+During density and reliability testing of OpenStack with Rally
+we observed test cases, during execution of which performance
+of OpenStack cluster has been drammatically degradated.
+
+Proposed change
+===============
+
+Develop a new Rally SLA plugin: *performance_degradation*
+
+This SLA plugin should find minimum and maximum duration of
+iterations completed without errors during Rally task execution.
+Assuming that minimum duration is 100%, it should calculate
+performance degradation against maximum duration.
+
+SLA plugin results:
+  - failure if performance degradation is more than value set
+  in plugin's max_degradation parameter;
+  - success if degradation is less
+  - performance degradation value as a percentage.
+
+How to enable this plugin:
+
+.. code:: json
+
+    "sla": {
+        "performance_degradation": {
+            "max_degradation": 50
+            }
+    }
+
+Alternatives
+------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+
+anevenchannyy <anevenchannyy@mirantis.com>
+
+Work Items
+----------
+
+ - Implement plugin
+ - Add non-voting job with this plugin to the most important OpenStack services
+
+Dependencies
+============
+
+None