OPENSTACK-2959: get from_device and to_device

  1. send the source device info to agent, agent can use the device
     info to delete source device neutorn selfip port.

  2. not raise exception when migrate between the same device, although
     it will cause neutron selfip port lost.

  3. when source device and target device is the same, we raise exception
     at neutron lbaas server side.

Author: zhang-shengping

f5lbaasdriver/v2/bigip/driver_v2.py

@@ -437,9 +437,14 @@ def _schedule_migrate_agent_device(
             LOG.info("Loadbalancer %s is rescheduled to agent %s",
                      loadbalancer.id, agent.id)
 
-        # Load device info and return
-        device = self.driver.device_scheduler.load_device(context,
-                                                          device_id)
+        # Load source device info
+        from_device = self.driver.device_scheduler.load_device(
+            context, bond['device_id'])
+
+        # Load target device info
+        device = self.driver.device_scheduler.load_device(
+            context, device_id)
+
         self._validate_device(device, loadbalancer, device_id)
 
         # Update binding table
@@ -451,8 +456,11 @@ def _schedule_migrate_agent_device(
             LOG.info("Loadbalancer %s is migrate to device %s",
                      loadbalancer.id, device["id"])
 
-        LOG.info("LB %s is migrate to agent %s device %s",
-                 loadbalancer.id, agent["id"], device["id"])
+        LOG.info("Loadbalancer %s is migrate from device %s to device %s "
+                 "by agent %s", loadbalancer.id, from_device["id"],
+                 device["id"], agent["id"])
+
+        device["from_device"] = from_device
 
         return agent, device