Ignore unconfigured machines from Vagrant env

Because `env.active_machines` can return machines no longer present in the Vagrantfile ([doc](http://rubydoc.info/github/mitchellh/vagrant/master/Vagrant/Environment:active_machines)), an "orphan" machine will not be configured and `env.machine` will throw `Vagrant::Errors::MachineNotFound`. It then prints this error message: "The machine with the name '...' was not found configured for this Vagrant
environment".

It can happen when having a multi-nodes Vagrantfile and playing with adding and removing node definitions.