Add support to `ceph-salt export` to write to a file directly

Currently, `ceph-salt export` dumps the content of the configuration to STDOUT by default and the user needs to redirect the output into a file themselves. Depending on the user's `umask`, this might accidentally expose the file's content to other users, which might be undesirable based on the information it contains, e.g. an SSH private key. (see https://github.com/SUSE/doc-ses/issues/339 for details).

It would be nice if the export could be written into a file directly, e.g. by passing the file name as an argument: `ceph-salt export `.

Start by locating the ceph-salt export command and how it currently writes configuration to STDOUT. Add support for a filename argument, then verify that ceph-salt export writes the configuration directly to that file instead of requiring shell redirection.

Written by the indexing model from the issue text.