fix #4808: ceph: mds create: use snake_case when setting options

fix #4808: ceph: mds create: use snake_case when setting options

As suggested in [1], it is recommended to use `_` in all cases when
dealing with config files. Note that this is for creation only, and we
enforce that there cannot be an existing MDS with the same ID, so we
do not have to bother how ceph would handle the case where both exist.

[1] https://docs.ceph.com/en/reef/rados/configuration/ceph-conf/#option-names

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>