Do not use `os.rename()`

This changes all occurrences of the [`os.rename`](http://docs.python.org/3/library/os.html#os.rename) function in the remote-host helper methods (file `remotes.py`) to use [`shutil.move`](http://docs.python.org/3/library/shutil.html#shutil.move) instead.

## The issue

When `os.rename` is used in combination with `tempfile.NamedTemporaryFile`, it tries to rename the temporary file with the name of the final destination file (usually in `/var/lib/ceph` or `/etc/ceph`). However, `os.rename` requires both files to be in the same filesystem.

A lot of systems are configured to mount dedicated filesystems distinct from the root filesystem (for instance for `/tmp` and `/var`, because those are used dynamically by the system). Consequently, commands like `ceph osd prepare` fail with the message: **[Errno 18] Invalid cross-device link**

## The fix

Fortunately, Python provides the `shutil.move` function:
* if both files are on the same filesystem, it defaults to `os.rename`
* otherwise, it copies the file to the new filesystem, then deletes the old file

This has been tested and seems working. No side-effect was observed.