title: Jenkins job wait for multiple open executor slots
body: <p>I have a generic job 'build' which is called by a concrete build for each codeline, e.g. 'stable_build'.  I have stable_build trigger build and have 'Block until the triggered projects finish their builds' checked, so that stable_build's result will mirror build's result.  This means that whenever I execute stable_build, it requires 2 executor slots, one for stable_build and one for build.  I have another job 'stable_tests' which triggers 'tests' in a similar fashion.  The problem I'm facing is this: if stable_build and stable_tests are executed simultaneously, each will take up an executor slot while the downstream 'build' and 'tests' are in the queue.</p>
  
  <p>Is there a way in Jenkins to configure a job to wait for 2 open executor slots, or perhaps another workaround?</p>