Update centos-art.sh script's tuneup module.
    
    - Previously, web pages with a html preamble in it end up with it
      duplicated. This commit adds sed commands to cleanup-before.sed file
      to remove HTML preamble from files before execute xmllint. This way
      the only the HTML preamble set by xmllint will remain.
    
    - Previously, the toc.awk file was unreachable because a wrong
      variable name. This made the table of content not to be generated as
      expected. This commit sets the correct variable name.
    
    - Previously, the tuneup module ended with an error because a wrong
      function name. This commit sets the correct function name and tuneup
      module ends without errors.