fix #1950: use /etc/pve/ceph.conf as default

fix #1950: use /etc/pve/ceph.conf as default

If there is no symlink between '/etc/ceph/ceph.conf -> /etc/pve/ceph.conf',
executing a RADOS command results in an exception.
For example on a cluster node where no monitor is configured and no
'pveceph init' was performed, this leads to unexpected conditions. On such a
node, if you check the monitor tab in GUI the monitors are correctly displayed,
but they seem to have no quorum, because actually the RADOS command didn't return.

Signed-off-by: Tim Marx <t.marx@proxmox.com>