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.
I suspect that it will be highly unlikely to have a device in any other place than `/dev/` but it still seems that this might not be robust enough.

`ceph-disk` does a thorough check for devices/partitions, I am not saying we should implement it exactly like this but just to give you an idea of what we should be looking for (as opposed to matching a string of a path):

    dev = os.path.realpath(dev)
    if not stat.S_ISBLK(os.lstat(dev).st_mode):
        raise Error('not a block device', dev)

    name = get_dev_name(dev)
    if os.path.exists(os.path.join('/sys/block', name)):
        return False

    # make sure it is a partition of something else
    for basename in os.listdir('/sys/block'):
        if os.path.exists(os.path.join('/sys/block', basename, name)):
            return True

    raise Error('not a disk or partition', dev)