global variable 'cpuOut' is always nil

```
package service

...
var cpuOut *os.File
...

// BeforeRun takes care of necessary actions such as creating files
// before the beat should run.
func BeforeRun() {
logger := logp.NewLogger("service")
if withCPUProfile() {
cpuOut, err := os.Create(*cpuprofile)
....
}

...
}
```

`cpuOut, err := os.Create(*cpuprofile)` will create a new variable `cpuOut`, which is different with global variable `var cpuOut *os.File`. Global variable `var cpuOut *os.File` is never set and one error will be created when function `Cleanup()` is called.

```
func Cleanup() {
logger := logp.NewLogger("service")
if withCPUProfile() {
pprof.StopCPUProfile()
cpuOut.Close() // -------- cpuOut is nil
}

...
}
```
Besides, the `cpuprofile` will not be closed in time.