Add an option for the bridge interface to drop the default route via the NAT interface (debian)

This adds an option, :drop_nat_interface_default_route, to the bridged virtualbox configuration hash. If enabled the bridged interface will have the following options set in /etc/network/interfaces:

    post-up route del default if eth0
    pre-down route add default if eth0

This has the effect that all traffic, apart from vagrant's ssh via the NAT, will go over the bridge, rather than the NAT, once it has come up.

I have found the default route via the NAT problematic when operating in networks with multiple subnets. If I try to point someone on another subnet at an application accessible via my bridged interface it will fail because the return traffic attempts to route via the wrong interface.