Static neighbor support

This changes adds support for assigning static ARP / NDP
neighbors on each ethernet interface.

Tested:
    Ran inside a romulus VM and made sure that `ip neigh add`
    commands triggered the refresh of static_neighbor objects.
    Also verified that static neighbors could be added through
    the CreateStatic dbus interface. Verified that deleting those
    static entries would persist to the system.

Change-Id: Ifaf753525c532b6dade392d7eb4ebd6d7242d480
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/network_manager.cpp b/network_manager.cpp
index fa5da0f..043d7a2 100644
--- a/network_manager.cpp
+++ b/network_manager.cpp
@@ -150,6 +150,7 @@
             bus, objPath.string(), dhcp, *this);
 
         intf->createIPAddressObjects();
+        intf->createStaticNeighborObjects();
 
         this->interfaces.emplace(
             std::make_pair(std::move(interface), std::move(intf)));