ceph: add MDS create/delete/list API

ceph: add MDS create/delete/list API

Allow to create, list and destroy and Ceph Metadata Server (MDS) over
the API and the CLI `pveceph` tool.

Besides setting up the local systemd service template and the MDS
data directory we also add a reference to the MDS in the ceph.conf
We note the backing host (node) from the respective MDS and set up a
'mds standby for name' = 'pve' so that the PVE created ones are a
single group. If we decide to add integration for rank/path specific
MDS (possible useful for CephFS with quite a bit of load) then this
may help as a starting point.

On create, check early if a reference already exists in ceph.conf and
abort in that case. If we only see existing data directories later
on we abort but do not remove them, they could well be from an older
manual create - where it's possible dangerous to just remove it. Let
the user handle it themself in that case.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Co-authored-by: Alwin Antreich <a.antreich@proxmox.com>