Fix hostname update on CentOS 6

The actual code fails with the following error on centos 6:
```
$ ansible xxxxxx -m hostname -a 'name=toto'
195.154.117.15 | FAILED >> {
    "failed": true, 
    "msg": "Command failed rc=127, out=, err=/bin/sh: hostnamectl: command not found\n"
}
```