Allow a MAC address to be specified for bridges

lib/vagrant/action/vm/bridged_network.rb

@@ -55,7 +55,10 @@ module Vagrant
             if type == :bridged
               options = args[0] || {}
 
-              results << { :adapter => 2 }.merge(options)
+              results << {
+                :adapter => 2,
+                :mac     => nil
+              }.merge(options)
             end
           end
 
@@ -86,7 +89,8 @@ module Vagrant
             adapters << {
               :adapter     => options[:adapter] + 1,
               :type        => :bridged,
-              :bridge  => options[:bridge]
+              :bridge      => options[:bridge],
+              :mac_address => options[:mac]
             }
           end