Add rally performance gate job for keystone

This patch is init setup of rally gate job.
After merging this everybody will be able to change
rally-scenarios/keystone.yaml to run any benchmarks with any load in gates.

Change-Id: Iae40d2550d797d013cf5e379117920c84adac6ac

rally-scenarios/README.rst

@@ -0,0 +1,5 @@
+This directory contains rally benchmark scenarios to be run by OpenStack CI.
+
+
+* more about rally: https://wiki.openstack.org/wiki/Rally
+* how to add rally-gates: https://wiki.openstack.org/wiki/Rally/RallyGates

rally-scenarios/keystone.yaml

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+---
+  KeystoneBasic.create_delete_user:
+    -
+      args:
+        name_length: 10
+      runner:
+        type: "constant"
+        times: 2500
+        concurrency: 60