Ignore unconfigured machines from Vagrant env

Because `#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.
Merge pull request #34 from Smartesting/fix_machine_not_found_error

Ignore unconfigured machines from Vagrant env