AMQP Connection is started even if "command" executor is used

When running the example:

https://github.com/dice-cyfronet/hyperflow/tree/feature/mm-example/examples/SplitAndGzip

I get

```
Done processing workflow JSON.
createInstance time: 10ms
Wf id=1
[AMQP] Starting connection!
...

Potentially unhandled rejection [1] TypeError: Object #<Promise> has no method 'once'
    at taskCount (/git/hyperflow-mm-example/functions/amqpCommand.js:12:14)
```
