Add enable_amqp parameter. Resolves #57

It appears to me that it's not the unavailability that we want to catch. I think the smarter way is to allow the app to be configured for ignoring the AMQP layer at all, this way I can extract the ```enable_amqp``` parameter into an environment specific config file depending on whether I need the queue:

In ```config.yml```:
```yaml
old_sound_rabbit_mq:
    enable_amqp:       %enable_amqp%
```

In ```parameters.yml``` (this file is in ```.gitignore``` usually):
```yaml
parameters:
    enable_amqp: false
```

Parameter name can surely be changed to your liking. This affects the env-specific config file as well as the DI extension.