conf: Requires either uuid or usage of secret

conf: Requires either uuid or usage of secret

As the RNG schema for disk auth secret implies, it requires either
"uuid" or "usage":

  <define name='diskAuthSecret'>
    <element name='secret'>
      <attribute name='type'>
        <choice>
          <value>ceph</value>
          <value>iscsi</value>
        </choice>
      </attribute>
      <choice>
        <attribute name='uuid'>
          <ref name="UUID"/>
        </attribute>
        <attribute name='usage'>
          <ref name='genericName'/>
        </attribute>
      </choice>
    </element>
  </define>