pytest: Add test cases for configuration validation
    
    Test cases cover configuration validation for sssd.conf,
    "config-check" option of the "sssctl" command to locate
    problems in the configuration file.
    
    Signed-off-by: Madhuri Upadhye <mupadhye@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>