Add sample for the server block migration scenario

Added a sample block migration for an already existing 
live migration scenario.

Change-Id: I7438358052c2408f1447d816a5603605131aee8d

samples/tasks/scenarios/nova/boot-and-block-migrate.json

@@ -0,0 +1,26 @@
+{
+   "NovaServers.boot_and_live_migrate_server": [
+        {
+            "args": {
+                "flavor": {
+                    "name": "m1.nano"
+                },
+                "image": {
+                    "name": "^cirros.*uec$"
+                },
+                "block_migration": true
+            },
+            "runner": {
+                "type": "constant",
+                "times": 10,
+                "concurrency": 2
+            },
+            "context": {
+                "users": {
+                    "tenants": 1,
+                    "users_per_tenant": 1
+                }
+            }
+        }
+    ]
+}

samples/tasks/scenarios/nova/boot-and-block-migrate.yaml

@@ -0,0 +1,16 @@
+---
+NovaServers.boot_and_live_migrate_server:
+- args:
+    flavor:
+      name: m1.nano
+    image:
+      name: "^cirros.*uec$"
+    block_migration: true
+  runner:
+    type: constant
+    times: 10
+    concurrency: 2
+  context:
+    users:
+      tenants: 1
+      users_per_tenant: 1