Add chown ceph:ceph for /tmp/ceph.mon.keyring

Add chown ceph:ceph for /tmp/ceph.mon.keyring

The keyring will be 600 by default with the owner of the current user who issues the command, which will make command "sudo -u ceph ceph-mon --mkfs" fail since the user "ceph" don't have access to the keyring file.
Because of that, the mon data directory won't have keyring and when "sudo systemctl start ceph-mon" it will show error "unable to read magic from mon data"

Signed-off-by: Jeffrey Chu <peihuachu1112@gmail.com>