[ONOS-8051] Device connected via BGP always shown available and reachable

In the command console, when "devices" command is executed, the call is routed
to DeviceManager. The DeviceManager queries the Provider of device periodically,
to check the isReachable status. If isReachable is true, isAvailable is eventually
set to true. Conversely, if isReachable is false, isAvailable is eventually set
to false.

Currently, BgptopologyProvider returns hard-coded "true" in isReachable() call.

This fix solves this by querying the current status from deviceService.
Also, since there is no "isReachable" call in deviceService, isAvailable() is used.

Change-Id: Ie6f7245c22232e0f867346f83b5aadaaf504031e
diff --git a/providers/bgp/topology/src/main/java/org/onosproject/provider/bgp/topology/impl/BgpTopologyProvider.java b/providers/bgp/topology/src/main/java/org/onosproject/provider/bgp/topology/impl/BgpTopologyProvider.java
index 8439bb9..88aff2f 100644
--- a/providers/bgp/topology/src/main/java/org/onosproject/provider/bgp/topology/impl/BgpTopologyProvider.java
+++ b/providers/bgp/topology/src/main/java/org/onosproject/provider/bgp/topology/impl/BgpTopologyProvider.java
@@ -595,8 +595,7 @@
 
     @Override
     public boolean isReachable(DeviceId deviceId) {
-        // TODO Auto-generated method stub
-        return true;
+        return deviceService.isAvailable(deviceId);
     }
 
     @Override