QOS V2 Driver: Policy/rule update/create resulting in removal of rules

Issue Description:
-----------------
When ODL controller is disconnected (i.e ODL shutdown) and

1) if newchanges are done for policy name or rules
2) If any new policy and rules are created then
after reconnection (i.e restart of ODL) of ODL, only policy is getting synced with ODL and policy rule is missing in ODL data store

Note: This problem occurs only when ODL is disconnected. when ODL is connected then no issue is observed with policy/rule update OR create.

2018-07-04 01:10:14.166 DEBUG networking_odl.common.client [req-fe573334-4a8a-409b-80a7-9d40ef14b99f None None] Sending METHOD (put) URL (http://10.164.224.59:8181/controller/nb/v2/neutron/qos/policies/73f0ed89-ad73-4102-8b74-a8e3bfde8833) JSON ({
  "policy": {
    "name": "bw-limiter6",
    "tags": [],
    "rules": [
      {
        "max_kbps": 8900,
        "direction": "egress",
        "qos_policy_id": "73f0ed89-ad73-4102-8b74-a8e3bfde8833",
        "type": "bandwidth_limit",
        "id": "4c752ea1-8382-4b4f-b435-f2bc91bcbdd1",
        "max_burst_kbps": 600
      }
    ],
    "tenant_id": "72236739e5844278b82ae4ad118e2e07",
    "created_at": "2018-07-03T19:26:37Z",
    "updated_at": "2018-07-03T19:28:12Z",
    "is_default": false,
    "revision_number": 2,
    "shared": false,
    "project_id": "72236739e5844278b82ae4ad118e2e07",
    "id": "73f0ed89-ad73-4102-8b74-a8e3bfde8833",
    "description": ""
  }
}) from (pid=28765) request /opt/stack/networking-odl/networking_odl/common/client.py:89
2018-07-04 01:10:14.330 DEBUG networking_odl.common.client [req-fe573334-4a8a-409b-80a7-9d40ef14b99f None None] Got response:
({
   "policy" : {
      "id" : "73f0ed89-ad73-4102-8b74-a8e3bfde8833",
      "tenant_id" : "72236739e5844278b82ae4ad118e2e07",
      "project_id" : "72236739e5844278b82ae4ad118e2e07",
      "revision_number" : 2,
      "name" : "bw-limiter6",
      "shared" : false
   }
}) from (pid=28765) _check_response /opt/stack/networking-odl/networking_odl/common/client.py:143

Steps to Reproduce:
---------------------

1.
openstack network qos policy create bw-limiter6

openstack network qos rule create --type bandwidth-limit --max-kbps 6600 --max-burst-kbits 600 --egress bw-limiter6

2. Verify the Policy and Rule are reflecting in ODL data store

3. Disconnect the ODL

4.openstack network qos rule set bw-limiter6 4c752ea1-8382-4b4f-b435-f2bc91bcbdd1 --max-kbps 8900

OR

openstack network qos policy set bw-limiter6 --name bw-limiter61

5. Reconnect the ODL

6. Verify the Qos policies in ODL data store and make sure changes are reflected and Policy rule present

Environment:
Devstack : Pike
ODL : Oxyagen