Provide a configuraiton in dea.yml to allow execute the hook scripts with priviledged enabled

Hi,

I am hoping to creae some directoreis in the /var/vcap before starting the application, with the before_start hook, I got the error for permission denied. Looks like the scripts are always run with vcap user. 

I am wondering whether we could add a new privileged option there, once configured, it is possible to run the scripts with 'root' user. I understood that, it could bring potential issues since the warden containers share some files with the host DEA VM. While considering that, only the platform operator could do that, it should be OK.

hook:
  priviledged: true
  before_start: /home/vcap/before_start.sh

Thoughts ? If it is reasonable, I could try to do the changes and creae a pull request.

Thanks.