Allow to use non-Str attributes as keys for members
    
    Locations use DNSNameParam as pkey_value, but implementation of searches
    for members was able to use only Str param. This commit allows to use
    other param classes for search.
    
    Required for: https://fedorahosted.org/freeipa/ticket/2008
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>