Automate required vagrant plugin installation for any number of plugins.

Had some time to kill: to save future users some typing.

In case vagrant needs more plugins in the future, one can simply add them to required_plugins in Vagrantfile and automate installation for all of them (given 'y').

Because vagrant doesn't seem to detect the additional installed plugins mid-run, one has to manually kill the current vagrant process and restart it with Ruby's exec.