Add a step in the wizard to ease LDAP configuration

It seems the documentation http://doc.nuxeo.com/display/ADMINDOC/Using+a+LDAP+Directory is not enough for about all (if not all) Nuxeo users.

Target configurations are (with membership relations):

1. Users from LDAP + groups stored in local SQL DB:
   • SQL groups reference LDAP users
2. Users and groups from LDAP :
   • LDAP groups reference LDAP users
3. Users from LDAP and SQL DB + groups stored in local DB :
   • SQL groups reference LDAP users
   • SQL groups reference SQL users
4. Users from LDAP + groups from LDAP or stored in local SQL DB:
   • SQL groups reference LDAP users
   • LDAP groups reference LDAP users
5. Users and groups from LDAP and stored in SQL DB :
   • SQL groups reference LDAP users
   • SQL groups reference SQL users
   • LDAP groups only reference LDAP users

In multi-directories configurations (3,4,5), creation, edition and deletion are only allowed for SQL users and groups