Use --data-dev or --data-dir for ceph-disk parameters to avoid making di...

ceph-disk parameters "--data-dev" or "--data-dir" are useful to avoid making directories such as /dev/sdc my mistake when you want to use a device that does not exist.
The one way to do this might be to check remotely if string starts with `/` and if that isn't a directory. For devices, python will report correctly, saying it is not a directory:

    $ python
    Python 2.7.3 (default, Sep 26 2013, 20:03:06)
    [GCC 4.6.3] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import os
    >>> os.path.isdir('/dev/sda')
    False
    >>> os.path.isdir('/dev/sda1')
    False
    >>> os.path.isdir('/tmp')
    True