opennebula: ds_mad can't be set for a system datastore

When we try to create ceph system datastore, the ds_mad attribute is set by default to 'ceph', while this attribute can't be set for a system datastore.
https://github.com/quattor/template-library-core/blob/03699fd060e7fc46b082f6a5537717531646fb25/components/opennebula/schema.pan#L280