Adds Designate server-list scenario

This submission adds method to
rally/benchmark/scenarios/designate/basic.py

1)def list_servers

Added configuration files for server operations under
samples/tasks/scenarios/designate

1) list-servers.json
2) list-servers.yaml
Also, added unittests to test_basic.py under
tests/unit/benchmark/scenarios/designate

Change-Id: I948f5e0c8beca963a9d6f1d388ea0410f2d107e3

rally-jobs/rally-designate.yaml

@@ -100,3 +100,18 @@
       sla:
         failure_rate:
           max: 0
+
+  DesignateBasic.list_servers:
+    -
+      runner:
+        type: "constant"
+        times: 4
+        concurrency: 2
+      context:
+        users:
+          tenants: 2
+          users_per_tenant: 2
+      sla:
+        failure_rate:
+          max: 0
+

samples/tasks/scenarios/designate/list-servers.json

@@ -0,0 +1,17 @@
+{
+    "DesignateBasic.list_servers": [
+        {
+            "runner": {
+                "type": "constant",
+                "times": 3,
+                "concurrency": 2
+            },
+            "context": {
+                "users": {
+                    "tenants": 2,
+                    "users_per_tenant": 2
+                }
+            }
+        }
+    ]
+}

samples/tasks/scenarios/designate/list-servers.yaml

@@ -0,0 +1,11 @@
+---
+  DesignateBasic.list_servers:
+    -
+      runner:
+        type: "constant"
+        times: 3
+        concurrency: 2
+      context:
+        users:
+          tenants: 2
+          users_per_tenant: 2