Add os-migration tests for Nova v3 API

v3 os-migration API returns a body which includes the info like:

 {
   "migrations": [
     {
       "id": 1,
       "status": "finished",
       "instance_uuid": "4ed20c05-6bcb-46c5-a6a4-115523660283",
       "source_node": "source-host",
       "source_compute": "source-host",
       "dest_node": "dest-host",
       "dest_compute": "dest-host",
       "dest_host": "10.21.43.148",
       "old_instance_type_id": 6,
       "new_instance_type_id": 7
       "created_at": "2014-02-17T18:59:18.000000",
       "updated_at": "2014-02-17T18:59:31.000000"
     }
   ]
 }

This patch adds this API tests.

Partially implements blueprint nova-v3-api-tests

Change-Id: I2bc8efa6eb5034ec2c4516fd147b8221600cf571
diff --git a/tempest/api/compute/base.py b/tempest/api/compute/base.py
index abd36a6..ac43869 100644
--- a/tempest/api/compute/base.py
+++ b/tempest/api/compute/base.py
@@ -339,6 +339,7 @@
         cls.hosts_client = cls.os.hosts_v3_client
         cls.quotas_client = cls.os.quotas_v3_client
         cls.version_client = cls.os.version_v3_client
+        cls.migrations_client = cls.os.migrations_v3_client
 
     @classmethod
     def create_image_from_server(cls, server_id, **kwargs):
@@ -412,3 +413,4 @@
         cls.hosts_admin_client = cls.os_adm.hosts_v3_client
         cls.quotas_admin_client = cls.os_adm.quotas_v3_client
         cls.agents_admin_client = cls.os_adm.agents_v3_client
+        cls.migrations_admin_client = cls.os_adm.migrations_v3_client