[VOL-2318] - Fix for jenkins NBI Failure

This commit adds the latest devices and logical devices to the
device agents and logical device agents respectively. Any GET
is returned directly from these agents instead from the model.
And any create/update results in the data being sent to the KV
store via the model and also results in the latest data in the
agent being updated.   If the Core dies and restart then the
latest data will be pulled from KV.   These changes assumes
that a device or a logical device is always owned by one Core
only which is the case.

Change-Id: Ie671cd70b38a58a3b32fa476eced5f218aeadad9
diff --git a/rw_core/coreif/logical_device_agent_if.go b/rw_core/coreif/logical_device_agent_if.go
index 34a71e5..167d309 100644
--- a/rw_core/coreif/logical_device_agent_if.go
+++ b/rw_core/coreif/logical_device_agent_if.go
@@ -27,7 +27,7 @@
 
 // LogicalDeviceAgent represents a generic agent
 type LogicalDeviceAgent interface {
-	GetLogicalDevice() (*voltha.LogicalDevice, error)
+	GetLogicalDevice() *voltha.LogicalDevice
 	GetDeviceGraph() *graph.DeviceGraph
 	GetWildcardInputPorts(excludePort ...uint32) []uint32
 	GetRoute(ingressPortNo uint32, egressPortNo uint32) []graph.RouteHop