The name of the unnumbered Interface which will be the nexthop for
this route. This member is complemented by vRtrStaticRouteNextHop.
Atmost only one of them can be non-null. If vRtrStaticRouteNextHop
is '0.0.0.0' and vRtrStaticRouteNextHopUnnumberedIf is a null
string then this static route is configured as a black hole.