Make IPAdmin not inherit from IPASimpleLDAPObject
    
    Instead of inheritance, the LDAPObject is kept in a `conn` attribute,
    as the ldap2 class does it.
    However, all IPASimpleLDAPObject methods are still available as before
    (using __getattr__-based proxying).
    
    Also, remove the IPAEntryLDAPObject class. The methods it overrode
    were never called in ways that would make them behave differently than
    the superclass.
    
    Part of the work for: https://fedorahosted.org/freeipa/ticket/2660