title: How to access VSTS Variable Groups from YAML build pipeline?
body: <p>I'm currently in the process of converting VSTS build pipelines to <a href="https://github.com/Microsoft/vsts-agent/blob/master/docs/preview/yamlgettingstarted.md" rel="nofollow noreferrer">YAML build definions</a>. However I can't yet find a way to use variables of a <a href="https://docs.microsoft.com/en-us/vsts/pipelines/library/variable-groups?view=vsts" rel="nofollow noreferrer">Variable Group</a> within my YAML file. If I reference a variable, just like before within a build pipeline, the PowerShell script doesn't seem to be able to find it.</p>
  
  <pre><code>- powershell: |
      Write-Output Testing...
      Write-Output "DatabaseName: $(AzureSqlDb_DatabaseName)"
    displayName: Output Library Vars
  </code></pre>
  
  <p>The detailed error message is as follows:</p>
  
  <pre><code>AzureSqlDb_DatabaseName : The term 'AzureSqlDb_DatabaseName' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
  At D:\a\_temp\e06f9ee7-8418-4339-a976-ed78a0e71021.ps1:3 char:31
  + Write-Output "DatabaseName: $(AzureSqlDb_DatabaseName)"
  +                               ~~~~~~~~~~~~~~~~~~~~~~~
      + CategoryInfo          : ObjectNotFound: (AzureSqlDb_DatabaseName:String) [], ParentContainsErrorRecordException
      + FullyQualifiedErrorId : CommandNotFoundException
  
  ##[debug]Exit code: 1
  ##[debug]Leaving Invoke-VstsTool.
  ##[error]PowerShell exited with code '1'.
  </code></pre>
  
  <p>As you can see, I've already enabled system.debug for the build pipeline. So:</p>
  
  <ul>
  <li>Is it already supported to use variable groups?</li>
  <li>If yes, what is the way to reference those variables?</li>
  </ul>