Fix : Error when -m is not present

When option -m is not present, we have this error : 

```
Traceback (most recent call last):
  File "./check_disks_by_ssh.py", line 154, in <module>
    mounts = opts.mounts.split(',')
AttributeError: 'NoneType' object has no attribute 'split'`
```

As, -m in not mandatory, this pull request seems to fix the error.