fixed creation of admin user

Trying to run a container results in the following errors:
```bash
cp: cannot stat `/var/lib/rabbitmq/mnesia/*': No such file or directory
/usr/lib/rabbitmq/bin/rabbitmq-server: 1: /usr/lib/rabbitmq/bin/rabbitmq-server: dirname: Permission denied
/usr/lib/rabbitmq/bin/rabbitmq-server: 20: .: Can't open /rabbitmq-env
/usr/lib/rabbitmq/bin/rabbitmqctl: 1: /usr/lib/rabbitmq/bin/rabbitmqctl: dirname: Permission denied
/usr/lib/rabbitmq/bin/rabbitmqctl: 20: .: Can't open /rabbitmq-env
/usr/lib/rabbitmq/bin/rabbitmqctl: 1: /usr/lib/rabbitmq/bin/rabbitmqctl: dirname: Permission denied
/usr/lib/rabbitmq/bin/rabbitmqctl: 20: .: Can't open /rabbitmq-env
/usr/lib/rabbitmq/bin/rabbitmqctl: 1: /usr/lib/rabbitmq/bin/rabbitmqctl: dirname: Permission denied
/usr/lib/rabbitmq/bin/rabbitmqctl: 20: .: Can't open /rabbitmq-env
/usr/lib/rabbitmq/bin/rabbitmqctl: 1: /usr/lib/rabbitmq/bin/rabbitmqctl: dirname: Permission denied
/usr/lib/rabbitmq/bin/rabbitmqctl: 20: .: Can't open /rabbitmq-env
/usr/lib/rabbitmq/bin/rabbitmqctl: 1: /usr/lib/rabbitmq/bin/rabbitmqctl: dirname: Permission denied
/usr/lib/rabbitmq/bin/rabbitmqctl: 20: .: Can't open /rabbitmq-env
/usr/lib/rabbitmq/bin/rabbitmq-server: 1: /usr/lib/rabbitmq/bin/rabbitmq-server: dirname: Permission denied
/usr/lib/rabbitmq/bin/rabbitmq-server: 20: .: Can't open /rabbitmq-env
```

It seemed to be using the wrong executable. So I just fixed the path and it worked properly. If I am not the only one around having that problem, we may could merge that PR. Cheers.