Fixed 'rabbitmqctl eval' command for old rabbitmq versions

[xiangyao1989]

This is a known issue in rabbitmq 3.3.4 or earlier, and the rabbitmqctl commands always appends ...done. at the end of outputs, which causes issues in cluster.rb when doing join to the cluster.
Please refer to my issue for details: #271

This fix force the command rabbitmqctl eval "node()." to always print out the first line which filters out the wrong tail string.

Before

[root@prod-rabbitmq-1001 ~]# rabbitmqctl eval "node()."
'rabbit@prod-rabbitmq-1001'
...done.

After

[root@prod-rabbitmq-1001 ~]# rabbitmqctl eval "node()." | head -1
'rabbit@prod-rabbitmq-1001'

I've tested this PR in the latest rabbitmq 3.5.3 and old versions like 3.3.4 and 3.2.3, and everything looks fine.

Thanks!

[michaelklishin]

👍