Delegate directive value quoting/unquoting to separate functions
    
    Separate functions were added to installutils module to quote/unquote a
    string in arbitrary characters.
    
    `installutils.get/set_directive` functions will use them to enclose
    the directive values in double quotes/strip the double quotes from
    retrieved values to maintain the original behavior.
    
    These functions can be used also for custom quoting/unquoting of
    retrieved values when desired.
    
    https://fedorahosted.org/freeipa/ticket/6991
    
    Reviewed-By: Tomas Krizek <tkrizek@redhat.com>