Allow non privileged user vagrant to run a shell provision

Sometimes, after provisioning the server, I need to make some initial configurations in the project, without superuser privileges. For example, bundle the project gems, import assets and/or db data from the integration server, etc. Some of these tasks are better performed by the user vagrant, and some of them don't even work when performed by root (because of ssh agent forwarding lost after sudo'ing and su'ing).

This pull request allows a shell provision by the unprivileged vagrant user:

    config.vm.provision :shell, :path => 'path/to/my/script', :privileged => false