make 'ctrl' an array (so that .each works)

ctrl should be an array since later on it is called with .each
Otherwise, my run complains with:

bin/openstackg5k:180:in `block (2 levels) in execute': undefined method `each' for "graphene-101-kavlan-4.nancy.grid5000.fr":String (NoMethodError)
        from /usr/lib/ruby/vendor_ruby/net/ssh/multi/session.rb:222:in `group'
        from bin/openstackg5k:179:in `block in execute'
        from /usr/lib/ruby/vendor_ruby/net/ssh/multi.rb:62:in `start'
        from bin/openstackg5k:170:in `execute'
        from bin/openstackg5k:240:in `educational'
        from bin/openstackg5k:155:in `runos'
        from bin/openstackg5k:339:in `<main>'