Network: Implement ipv6/ipv4 default gateways on EthernetInterface

Currently ipv6/ipv4 default gateway properties on SystemConfiguration
interface which will be deprecated soon.
This commit is to add Default gateway properties implementation on
EthernetInterface.

Test by:
Set ipv4/v6 default gateway properties on ethernet interface.

Signed-off-by: Ravi Teja <raviteja28031990@gmail.com>
Change-Id: I30da81d7ccf9d568a60bfdd179331b4c0d86b149
diff --git a/test/test_ethernet_interface.cpp b/test/test_ethernet_interface.cpp
index ab2279f..67a26dc 100644
--- a/test/test_ethernet_interface.cpp
+++ b/test/test_ethernet_interface.cpp
@@ -204,5 +204,19 @@
     EXPECT_EQ(servers, values);
 }
 
+TEST_F(TestEthernetInterface, addGateway)
+{
+    std::string gateway = "10.3.3.3";
+    interface.defaultGateway(gateway);
+    EXPECT_EQ(interface.defaultGateway(), gateway);
+}
+
+TEST_F(TestEthernetInterface, addGateway6)
+{
+    std::string gateway6 = "ffff:ffff:ffff:fe80::1";
+    interface.defaultGateway6(gateway6);
+    EXPECT_EQ(interface.defaultGateway6(), gateway6);
+}
+
 } // namespace network
 } // namespace phosphor