title: How can I sum multiple columns in a spark dataframe in pyspark?
body: <p>I've got a list of column names I want to sum</p>
  <pre><code>columns = ['col1','col2','col3']
  </code></pre>
  <p>How can I add the three and put it in a new column ? (in an automatic way, so that I can change the column list and have new results)</p>
  <p>Dataframe with result I want:</p>
  <pre><code>col1   col2   col3   result
   1      2      3       6
  </code></pre>