title: Sonarqube set leak period to specific version other than previous version
body: <p>Evaluating Sonarqube (Version 5.4), we want to migrate our current workflow <br>using another Audit tool, which works like that :</p>
  
  <p>The current version that runs in production is our reference version.<br>
  A new development version is checked out from GIT, a diff process calculates the new and modified files vs.
  the reference version and starts the audit for these files.<br>
  There's also a slightly different handling of legacy code (components that already existed in 2012) and
  new components (after 2012).</p>
  
  <p>The build breaks if :</p>
  
  <p>Blocker issues in changed files (those files already existing in 2012) from legacy components<br>
  Blocker or critical issues in new files (files created after 2012) from legacy and new components</p>
  
  <p>How to implement that in Sonarqube ?</p>
  
  <p>Tried two things already :<br></p>
  
  <p>1.) Set property <code>sonar.timemachine.period1</code> to the production/reference version in Ant script before starting Sonar task => didn't work,
  it's always 'since previous version'</p>
  
  <p>2.) Define two different projects in Sonarqube, one for the production versions and one for the new dev versions.
  Then programmatically use the feature known from Sonarqube Web UI More / Compare Projects and get the diff
  for Blocker and Critcal issues.</p>
  
  <p>Problem : f.e. i'll get no diff for Critical issues if i have fixed 200 Critical issues that already existed in my production
  reference, but introduced 200 new issues in the development version.<br>
  The Compare Projects feature has no metric for new or old issues, it's just counting issues for the  compared projects.</p>