split horizon bit should be reset to zero for centralized use cases (SNAT), to support SNAT through VLAN/Flat private network

Following this commit - https://git.opendaylight.org/gerrit/#/c/47794/ - there is now a split horizon protection in the Egress dispatcher (220).

This means traffic that originates from a VLAN/Flat network will not be outputted back to a VLAN/Flat network.
This needs to be overridden in specific usecases where our switches aren't distributed (places that use a centralized switch, e.g. NAPT switch).

Currently traffic originating from a compute that is NOT the NAPT switch, through a VLAN/Flat private network, will not be able to use the NAPT feature as the packet will be dropped before output on the NAPT switch.