Add ability to manually add ARP/ND entries

[cjdoucette]

We should be able to manually add ARP and ND entries to the LLS cache using the dynamic configuration block.

[AltraMayor]

A workaround is to ping the address to be added to the LLS cache from a KNI interface. This will force the resolution of the address and add it to the cache. It's not the same thing as directly editing the cache, though.