fix #3451 ui: ceph create osd: show custom device classes

fix #3451 ui: ceph create osd: show custom device classes

Showing already configured custom device classes makes it easier to
create new OSDs with custom device classes.

The Crush map contains a list of all OSDs in the cluster, including
their device class.
This means we can create a list of used device classes from it, avoiding
adding another API endpoint.

Fetching the crushmap should also be quite a bit less data that needs to
be transferred, compared to the other possible nodes/<node>/ceph/osd
endpoint, especially in larger clusters.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>