fix: use exec to properly handle signals for graceful shutdown

Use `exec` to execute RabbitMQ's server, so that the server process can
receive signals such as `SIGTERM`.

Closes reanahub/reana-job-controller#347

start.sh

@@ -14,4 +14,4 @@ EOF
 chown rabbitmq:rabbitmq /var/lib/rabbitmq/mnesia
 # Limit maximum number of open file descriptors to avoid high memory usage
 ulimit -n 1048576
-rabbitmq-server
+exec rabbitmq-server