commands/rdp: support username param [GH-5460]

plugins/commands/rdp/command.rb

@@ -80,6 +80,7 @@ module VagrantPlugins
 
         rdp_info[:host] ||= ssh_info[:host]
         rdp_info[:port] ||= machine.config.rdp.port
+        rdp_info[:username] ||= machine.config.rdp.username
 
         if rdp_info[:host] == "127.0.0.1"
           # We need to find a forwarded port...

plugins/commands/rdp/config.rb

@@ -3,15 +3,18 @@ module VagrantPlugins
     class Config < Vagrant.plugin("2", :config)
       attr_accessor :port
       attr_accessor :search_port
+      attr_accessor :username
 
       def initialize
         @port        = UNSET_VALUE
         @search_port = UNSET_VALUE
+        @username = UNSET_VALUE
       end
 
       def finalize!
         @port        = 3389 if @port == UNSET_VALUE
         @search_port = 3389 if @search_port == UNSET_VALUE
+        @username = nil if @username == UNSET_VALUE
       end
 
       def validate(machine)