remove world read permission for log files

As part of an investigation to make DISA-STIG recommendations for MicroCeph, our external consultant found this issue:

/var/snap/microceph/common/log has 4 log files (ceph-mds*, ceph-mon*, ceph-mgr*, and ceph.osd*) that have world readable permissions (644). Those should be changed to 640 permissions.

Start by locating how /var/snap/microceph/common/log creates ceph-mds*, ceph-mon*, ceph-mgr*, and ceph.osd* log files. Confirm that each file is created with 640 permissions rather than 644, then verify the resulting permissions for all four log-file patterns.

Written by the indexing model from the issue text.