leap_cli: allow for user-specified additional vagrant options

Due to working on #861 (closed), I locked myself out of the vpn1 vagrant node in the middle of a leap deploy because the firewall came up and shut everything down. That was interesting... I couldn't 'leap ssh' into the node either. In order to get at it, I needed to make a change to the VagrantFile to enable gui support so I could login to the console. Unfortunately, any change to the VagrantFile gets overwritten on every 'leap local' command. So I had to go into the leap_cli source and modify the vagrant.rb to add a line in there, recompile leap_cli, reinstall.

It would be nice if there was a way a user could specify additional lines to VagrantFile, as I could imagine other things like this happening.

In particular, the line I had to add was:

config.vm.boot_mode = :gui

it isn't something that should be enabled at all times, because when it is there, it pops up a gui console when the vagrant node is started.

(from redmine: created on 2013-01-24, closed on 2013-12-04, relates #861 (closed))