Allow controller to set MAC address to use in ARP responses for SNAT IPs.

Allow controller to set MAC address to use in ARP responses for SNAT IPs.

This allows the controller to set a MAC address to use in response to
an ARP request for the NAT IP address on a non-NAT interface.  This is
useful if a NAT'd device needs to communicate with a non-NAT'd device,
when they are on the same interface on the OpenFlow switch.  When the
non-NAT'd device requests the MAC address of the NAT IP address, the
switch responds with the supplied MAC address (often the L3 router
behind it).  This allows communication in both directions to bounce off
the L3 router and not confuse controller.