Networking_odl full_sync record l2gw connection creation journal entry with wrong key name.

・Summary : Networking_odl full_sync record l2gw connection creation journal entry with wrong key name.

・Version: stable/ocata & master

・Issue faced :
  We found that l2gw connection is not created sucessfully after full_sync executed.
  By checking both neutron debug log and ODL karaf log, we found the cause is that json data send to ODL for l2gwconnection creation generated by full sync is different with json data generated by new neutron creation command.
  For neutron creation command, journal entry is recorded by l2gw v2 driver after neutron DB updated.(using key name "gateway_id")
  https://github.com/openstack/networking-odl/blob/master/networking_odl/l2gateway/driver_v2.py#L82
  For full sync, journal entry is recorded directly by full_sync without l2gw v2 driver.(using key name "l2_gateway_id")

  Below are neutron logs.
  Neutron creation command：
  2017-09-04 17:33:37.037 28382 DEBUG networking_odl.common.client [-] Sending METHOD (post) URL (http://172.16.1.30:8080/controller/nb/v2/neutron/l2gateway-connections) JSON ({
    "l2gateway_connection": {
      "gateway_id": "8b35c5a9-0498-4754-aa25-f824210607c4",
      "network_id": "78224892-3cb0-47f9-8879-858f8072bef9",
      "tenant_id": "e309f6c154ef44f7bf839cd653069087",
      "segmentation_id": "2222",
      "project_id": "e309f6c154ef44f7bf839cd653069087",
      "id": "f4805b7e-784e-4119-b940-049be9e438ba"
    }
  }) request /usr/lib/python2.7/site-packages/networking_odl/common/client.py:86
  2017-09-04 17:33:37.149 28382 DEBUG networking_odl.common.client [-] Got response:
  ({
     "l2gateway_connection" : {
        "id" : "f4805b7e-784e-4119-b940-049be9e438ba",
        "tenant_id" : "e309f6c154ef44f7bf839cd653069087",
        "project_id" : "e309f6c154ef44f7bf839cd653069087",
        "gateway_id" : "8b35c5a9-0498-4754-aa25-f824210607c4",
        "network_id" : "78224892-3cb0-47f9-8879-858f8072bef9",
        "segmentation_id" : 2222
     }
  }) _check_rensponse /usr/lib/python2.7/site-packages/networking_odl/common/client.py:141

  full_sync：
  2017-09-04 12:09:11.735 31030 DEBUG networking_odl.common.client [-] Sending METHOD (post) URL (http://172.16.1.30:8080/controller/nb/v2/neutron/l2gateway-connections) JSON ({
    "l2gateway_connection": {
      "l2_gateway_id": "c3222ac9-c3ed-4e17-92e5-ba81c51252a2",
      "network_id": "043bd808-3c2b-4d59-a1e8-18e1d1c214f7",
      "tenant_id": "e309f6c154ef44f7bf839cd653069087",
      "segmentation_id": 2222,
      "project_id": "e309f6c154ef44f7bf839cd653069087",
      "id": "e8f142ca-26d4-44e9-80a8-1b5398519a4d"
    }
  }) request /usr/lib/python2.7/site-packages/networking_odl/common/client.py:86
  2017-09-04 12:09:12.254 31030 DEBUG networking_odl.common.client [-] Got response:
  ({
     "l2gateway_connection" : {
        "id" : "e8f142ca-26d4-44e9-80a8-1b5398519a4d",
        "tenant_id" : "e309f6c154ef44f7bf839cd653069087",
        "project_id" : "e309f6c154ef44f7bf839cd653069087",
        "network_id" : "043bd808-3c2b-4d59-a1e8-18e1d1c214f7",
        "segmentation_id" : 2222
     }
  }) _check_rensponse /usr/lib/python2.7/site-packages/networking_odl/common/client.py:141