Changed daemon command to use $ES_USER

I just installed `elasticsearch-0.90.0-1.noarch.rpm` and have not been able to start/stop elasticsearch using `service elasticsearch start`.

At line 67 where it tries to exec the `daemon` command, it is using a variable `$USER`, which is not set anywhere in the script, which causes that command to fail.

This should be changed to `$ES_USER`.