Adding vrde support for virtualbox-iso builder

VirtualBox has built-in support for Remote Desktop Protocol with the vboxmanage vrde flag (virtualbox remote desktop extensions). If the virtualbox remote desktop extensions pack is installed on the machine using packer, and if vrde is set to on, then users can connect to the vm using the hostname or ip address of the host operating system at the default port of 3899, using software compliant to Remote Desktop Protocol (e.g. Microsoft's Remote Desktop Connection). This will enable users to view and control the box using remote software, which is helpful when troubleshooting failed kickstarts. 

There are two booleans that can be set in the JSON file: vrde and clean_vrde. vrde will toggle support for vrde in the vm. clean_vrde will determine if vrde is kept on or turned off before generating the artifacts.