Add rally performance gate job for glance

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

Change-Id: Ic5e58856ece15e4d130ccd1e0057917441b01728

rally-scenarios/README.rst

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+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/glance.yaml

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+---
+  GlanceImages.create_and_list_image:
+    -
+      args:
+        image_location: "http://download.cirros-cloud.net/0.3.1/cirros-0.3.1-x86_64-disk.img"
+        container_format: "bare"
+        disk_format: "qcow2"
+      runner:
+        type: "constant"
+        times: 100
+        concurrency: 5
+      context:
+        users:
+          tenants: 1
+          users_per_tenant: 1
+
+  GlanceImages.create_and_delete_image:
+    -
+      args:
+        image_location: "http://download.cirros-cloud.net/0.3.1/cirros-0.3.1-x86_64-disk.img"
+        container_format: "bare"
+        disk_format: "qcow2"
+      runner:
+        type: "constant"
+        times: 100
+        concurrency: 5
+      context:
+        users:
+          tenants: 5
+          users_per_tenant: 2