Netgroup nisdomain and hosts validation
    
    nisdomain validation:
    Added pattern to the 'nisdomain' parameter to validate the specified
    nisdomain name. According to most common use cases the same pattern as
    for netgroup should fit. Unit-tests added.
    
    https://fedorahosted.org/freeipa/ticket/2448
    
    'add_external_pre_callback' function was created to allow validation of
    all external members. Validation is based on usage of objects primary
    key parameter. The 'add_external_pre_callback' fucntion has to be called
    directly from in the 'pre_callback' function. This change affects
    netgroup, hbacrule and sudorule commands.
    
    For hostname, the validator allows non-fqdn and underscore characters.
    validate_hostname function in ipalib.util was modified and contains
    additional option that allows hostname to contain underscore characters.
    This option is disabled by default.
    
    Unit-tests added.
    
    https://fedorahosted.org/freeipa/ticket/2447