Make DHCP IP configurable

This makes it possible to have DHCP attribute IPs to guests under
various subnets

plugins/providers/virtualbox/action/network.rb

@@ -249,14 +249,16 @@ module VagrantPlugins
             auto_config: true,
             mac:         nil,
             nic_type:    nil,
-            type:        :static
+            type:        :static,
+            dhcp_ip:     "172.28.128.1"
           }.merge(options)
 
           # Make sure the type is a symbol
           options[:type] = options[:type].to_sym
 
           # Default IP is in the 20-bit private network block for DHCP based networks
-          options[:ip] = "172.28.128.1" if options[:type] == :dhcp && !options[:ip]
+          options[:ip] = options[:dhcp_ip] if options[:type] == :dhcp && !options[:ip]
+
 
           ip = IPAddr.new(options[:ip])
           if ip.ipv4?