* scripts/check_rabbitmq_watermark: check if memory used is below threshold

Hi!

I've added a check to be warned if RabbitMQ reaches 80%+ of the maximum allowed memory.

This allows to be notified by Nagios before RabbitMQ raises a memory alarm and blocks all connections, hence avoiding a deny of service.

This produces the following message:

```
./check_rabbitmq_watermark -u guest --extra-opts=rabbitmq@/tmp/1.ini
RABBITMQ_WATERMARK OK - 35742864 bytes used out of 5002548019 (0.714493171564696%) mem_alarm disk_free_alarm
```

Could you please merge this pull request?

Regards,