LinkDiscovery support for Arista.

Provide the method of link discovery for legacy network.
Openflow provide the link discovery information itself.
But at legacy network side, it can be done by LLDP functionality.

So, this patch implements the gethering of LLDP information from arista switch
and constructs the information of LinkDescription for onos.

So that, onos can recognize the link status and utilize it.

ONOS-7707

Change-Id: I049fe4c0a5d4bf3bf78e985800941d0e131647e1
diff --git a/drivers/arista/src/main/resources/arista-drivers.xml b/drivers/arista/src/main/resources/arista-drivers.xml
index 15b42be..91238a9 100644
--- a/drivers/arista/src/main/resources/arista-drivers.xml
+++ b/drivers/arista/src/main/resources/arista-drivers.xml
@@ -20,6 +20,8 @@
                    impl="org.onosproject.drivers.arista.DeviceDescriptionDiscoveryAristaImpl"/>
         <behaviour api="org.onosproject.net.behaviour.ControllerConfig"
                    impl="org.onosproject.drivers.arista.ControllerConfigAristaImpl"/>
+        <behaviour api="org.onosproject.net.behaviour.LinkDiscovery"
+                   impl="org.onosproject.drivers.arista.LinkDiscoveryAristaImpl"/>
     </driver>
 </drivers>