mgr/cephadm: support non-root ssh-user w permissions

mgr/cephadm: support non-root ssh-user w permissions

Restructured code, so that in case of non-root, the resulting file will
be created with permissions set to the ssh-user. This allows the
subsequent scp to be able to write the file. The remaining code kept the
same, so that file permissions are restored to the expected ones, but
just runs after the scp.

Fixes: https://tracker.ceph.com/issues/54620
Signed-off-by: Christoph Glaubitz <c.glaubitz@syseleven.de>
(cherry picked from commit 452e52a7e39409e3409d59940133333416b830bc)