[spec] Add performance degradation SLA plugin
[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

vganapath/rally,gluke77/rally,openstack/rally,gluke77/rally,gluke77/rally,yeming233/rally,openstack/rally,openstack/rally,vganapath/rally,vganapath/rally,vganapath/rally,openstack/rally,gluke77/rally,yeming233/rally