[ONOS-4797] Implement the Driver for handling Arista switch

Arista switch with its recent network OS, EOS, supports remote configuration & control. The protocol for this is Arista's proprietary but REST style.
Since ONOS has already REST southbound protocol, the control of Arista switch can be easily done by using this.

- Implemnted driver Loader for Arista switch
- Implemented DeviceDescriptionDiscovery for Arista switch
- [Deleted] Implemented the method sending post to Arista switch and return the result as a String

Change-Id: I9d4b8f0d707361949ff530ff414adb0437ae87d9
diff --git a/BUCK b/BUCK
index d2e4aec..f473d55 100644
--- a/BUCK
+++ b/BUCK
@@ -55,6 +55,7 @@
     '//drivers/netconf:onos-drivers-netconf-oar',
     '//drivers/ovsdb:onos-drivers-ovsdb-oar',
     '//drivers/optical:onos-drivers-optical-oar',
+    '//drivers/arista:onos-drivers-arista-oar',
 
     '//providers/netconf/device:onos-providers-netconf-device',
     '//providers/openflow/device:onos-providers-openflow-device',