IPv6 gateway support

This implements the dbus interface plumbing needed to properly get and
set the defaultGateway6 property.

Tested:
    Made sure manual changes to the default route were discovered.
    Changed the route via the dbus interface and saw it write the
    correct entry to the .network file. Ensured that restarting the
    daemon and the BMC preserved the defaultGateway6 field.

Change-Id: Ida6a168e0df0a42b4e0d3f6d569ec2fe1f9adaab
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/system_configuration.hpp b/system_configuration.hpp
index 9e76dca..a29309c 100644
--- a/system_configuration.hpp
+++ b/system_configuration.hpp
@@ -45,13 +45,20 @@
      */
     std::string hostName(std::string name) override;
 
-    /** @brief set the default gateway of the system.
-     *  @param[in] gateway - default gateway of the system.
+    /** @brief set the default v4 gateway of the system.
+     *  @param[in] gateway - default v4 gateway of the system.
      */
     std::string defaultGateway(std::string gateway) override;
 
     using SystemConfigIntf::defaultGateway;
 
+    /** @brief set the default v6 gateway of the system.
+     *  @param[in] gateway - default v6 gateway of the system.
+     */
+    std::string defaultGateway6(std::string gateway) override;
+
+    using SystemConfigIntf::defaultGateway6;
+
   private:
     /** @brief get the hostname from the system by doing
      *         dbus call to hostnamed service.