title: airflow startup failed due to gunicorn
body: <pre><code>Traceback (most recent call last):
    File "./python_runtime/bin/airflow", line 15, in &lt;module&gt;
      args.func(args)
    File "/home/admin/airflow/python_runtime/lib/python2.7/site-packages/airflow/bin/cli.py", line 423, in webserver
      'gunicorn', run_args
    File "/home/admin/airflow/python_runtime/lib/python2.7/os.py", line 346, in execvp
      _execvpe(file, args)
    File "/home/admin/airflow/python_runtime/lib/python2.7/os.py", line 385, in _execvpe
      func(fullname, *argrest)
  OSError: [Errno 2] No such file or directory
  </code></pre>
  
  <p>I find out that this error was cause by no gunicorn avaiable,so I print all paths in os.py:</p>
  
  <pre><code>/home/admin/airflow/python_runtime/bin/gunicorn
  /home/admin/python2.7.2/bin/gunicorn
  /home/admin/tools/local/bin/gunicorn
  /home/admin/tools/bin/gunicorn
  /home/admin/tools/opsdb/gunicorn
  /home/admin/odps_tools/dship/gunicorn
  /home/admin/php/bin/gunicorn
  /home/admin/cmake/bin/gunicorn
  /home/admin/jdk1.6.0_24/bin/gunicorn
  /home/admin/tools/bin/gunicorn
  /usr/kerberos/bin/gunicorn
  /usr/local/sbin/gunicorn
  /usr/sbin/gunicorn
  /sbin/gunicorn
  /usr/local/bin/gunicorn
  /bin/gunicorn
  /usr/bin/gunicorn
  /usr/X11R6/bin/gunicorn
  </code></pre>
  
  <p>while gunicorn exist in this path</p>
  
  <pre><code>/home/admin/airflow/python_runtime/bin/gunicorn
  </code></pre>
  
  <p>something wrong with my env?</p>