Add user-specific Vagrantfile.
Don't automatically update VirtualBox Guest plugin with Vagrant.
`Vagrant.require_plugin` is deprecated.
Warn if the `vagrant-vbguest` plugin is missing.
Warn if the `vagrant-cachier` plugin is not installed.
Use NFS for `vagrant-cachier` plugin.
Use "box" scope for `vagrant-cachier`.
Warn if the `vagrant-hostmanager` plugin is not installed.
Configure the `vagrant-hostmanager` plugin.
Update submodules.