Configurable external gateway modes extension model. By default, when a gateway is attached to a router using the Neutron L3 extension,
Network Address Translation (NAT) is enabled for traffic generated by subnets attached to the router. With this extension, the user will
have the option of choosing whether SNAT should be enabled or not on a router basis.
SNAT can be enabled or disabled at any time on a Neutron router regardless of the current status of floating IPs.
Floating IPs will continue working even when SNAT is disabled.
NOTE: The default setting is True to ensure backward compatibility for plugins supporting this extension.