add use_dhcp_assigned_default_route setting to allow for dhcp bridged interfaces which have a default route assigned by dhcp

lib/vagrant/action/vm/network.rb

@@ -315,7 +315,8 @@ module Vagrant
             :adapter => nil,
             :mac     => nil,
             :bridge  => nil,
-            :auto_config => true
+            :auto_config => true,
+            :use_dhcp_assigned_default_route => false
           }.merge(options)
         end
 

templates/guests/debian/network_dhcp.erb

@@ -2,5 +2,7 @@
 # The contents below are automatically generated by Vagrant. Do not modify.
 auto eth<%= options[:interface] %>
 iface eth<%= options[:interface] %> inet dhcp
+<% unless options[:use_dhcp_assigned_default_route] %>
     post-up route del default dev $IFACE
+<% end %>
 #VAGRANT-END