Generalize AD GC search
    
    Modify access methods to AD GC so that callers can specify a custom
    basedn, filter, scope and attribute list, thus allowing it to perform
    any LDAP search.
    
    Error checking methodology in these functions was changed, so that it
    rather raises an exception with a desription instead of simply returning
    a None or False value which would made an investigation why something
    does not work much more difficult. External membership method in
    group-add-member command was updated to match this approach.
    
    https://fedorahosted.org/freeipa/ticket/2997