Fix automountkey-mod
    
    Fix automountkey-mod so that automountkey attribute is correctly
    updated. Add this test case to the unit tests.
    
    Make automountkey required for automountkey-mod, otherwise it would
    cause internal server error.
    
    Make --newinfo optional so that automountkey may be just renamed
    without changing its info attribute.
    
    https://fedorahosted.org/freeipa/ticket/1528