webui: attr widget: get list of possible attrs from ipapermdefaultattr
    
    Very useful for managed permissions since the list of attrs in metadata
    might be smaller that default attributes. This smooths behavior if one
    removes an attr from effective attrs which is not in metadata. Without
    this it will disappear from the list and one has to add it manually
    through 'Add'.
    
    https://fedorahosted.org/freeipa/ticket/4253
    
    Reviewed-By: Endi Sukma Dewata <edewata@redhat.com>