Fix idempotence in subsequent Chef runs

[tknerr]

Running the update-vm.sh multiple times kept reinstalling Vagrant, Vagrant plugins as well as VirtualBox even though it was already installed.

These were 3 different issues which broke idempotence:

1. the vagrant dpkg_package installation was missing a guard expression to check if the desired vagrant version is already installed
2. the install_vagrant_plugin(name, version) helper function actually had a guard, but it was no longer working due to a change in the output of vagrant plugin list that we parse
3. the VirtualBox installation was not idempotent because we introduced a version conflict by installing libcurl3 at the beginning of the Chef, run where the VirtualBox installation later on needs (and thus reinstalled) libcurl4

This PR fixes the issues above, so that we have proper idempotence restored and subsequent Chef runs no longer reinstall the already installed stuff