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

[LenzGr]

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 <filename>.