add debian/network_bridged.erb template

Having templates for "bridged" type networks, allows me to configure a default gateway on the bridged interface instead on the nat eth0.

for example, in my Vagrantfile:

  c.vm.network(:public_network, {
  	 :bridge => "en2: Thunderbolt Ethernet",
  	 :type => :bridged,
  	 :ip => "172.26.133.137", 
  	 :netmask => "255.255.255.192",
  	 :gateway => "172.26.133.129",
  	 :drop_nat_interface_default_route => true
  	})