Added reviewer role on admin groups.

This role ensures that admins can see the menu bar.

@phgross 
`opengever.setup` uses the `administrator.txt` to determine the admin groups. Should i use the same file for the upgrade-step? Or should i add the role to all groups with the `Administrator` role (group names may have changed..)?
Is this testable? It would require a full site installation, so i guess not.
use  `Products.PluggableAuthService.plugins.ZODBRoleManager.listAssignedPrincipals` instead of group loop.