title: sonar project.properties to ignore files containing a regular expression
body: <p>I am trying to exclude files containing generated code in a sonar project.properties file.
  I have added the following to <code>project.properties</code>.</p>
  
  <pre><code>sonar.issue.ignore.allfile=.*generated.*,.*GeneratedCodeAttribute.*
  </code></pre>
  
  <p>I get the following error:
  ERROR: Caused by: Exclusions > Issues : Invalid format. The field does not define a regular expression: null</p>
  
  <p>I have tried a lot of combinations. None of these work, same error.</p>
  
  <pre><code>sonar.issue.ignore.allfile=file:generated
  sonar.issue.ignore.allfile=generated
  sonar.issue.ignore.allfile=regex:generated
  
  etc...
  </code></pre>
  
  <p>Any ideas on how to set this property in a <code>project.properties</code> file?<br>
  I am aware that this can be done in the user interface, I need the project file syntax for multiple exclusions. I have read the source code, but cannot figure out what's missing.</p>