Fine tune the role for cASO (#642)

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# Summary

Be more precise on the role needed for cASO

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---

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**Related issue :**

content/en/providers/cloud-compute/openstack/vo-config/_index.md

@@ -109,11 +109,11 @@ Add the project supporting the VO to cASO:
    }
    ```
 
-Be sure to include the user running cASO as member of the project if it does not
-have admin privileges:
+Be sure to include the user running cASO at least as `reader` of the project if
+it does not have admin privileges:
 
 ```shell
-openstack role add member --user <your caso user> --project <your new vo project>
+openstack role add --user <your caso user> --project <your new vo project> reader
 ```
 
 ## Information system / VM Image management