Make `vagrant up` work even if network is unavailable

Nobody has claimed this yet.

At the moment running `vagrant up` even on a box that is already provisioned on a computer that doesn't have any network access will result in an error because Vagrant cannot check for box updates.

We could add the following configuration option to the `Vagrantfile` to prevent box update checking: `config.vm.box_check_update = false`. It would be of course better that Vagrant doesn't completely fail but just warns about not being able to check for updates, but that's apparently not possible.

Start in the Vagrantfile and review the existing VM configuration. Test `vagrant up` with the box already provisioned and network access unavailable. Done means the update check is disabled and the command no longer fails in that offline scenario.

Written by the indexing model from the issue text.