title: Airflow webserver doesnt start except in debug mode
body: <p>Airflow webserver start only in debug mode:</p>
  
  <p><code>airflow webserver -p 8051</code></p>
  
  <p>And the <code>traceback</code> I get:</p>
  
  <pre><code>[2016-12-07 03:38:48,056] {__init__.py:36} INFO - Using executor CeleryExecutor
  [2016-12-07 03:38:48,056] {models.py:154} INFO - Filling up the DagBag from  /home/user/some_dir/airflow/dags. Running the Gunicorn server with 4 syncworkers on host 0.0.0.0 and port 8051 with a timeout of 120... 
  ['gunicorn', '-w 4', '-k sync', '-t 120', '-b 0.0.0.0:8051', '-n airflow-  webserver', '-p /home/user/some_dir/airflow/airflow-webserver.pid', 'airflow.www.app:cached_app()']
  Traceback (most recent call last):
  File "./airflow", line 15, in &lt;module&gt; args.func(args)
  File "/home/user/userenv/local/lib/python2.7/site-packages/airflow/bin/cli.py", line 426, in webserver 'gunicorn', run_args
  File "/home/user/userenv/lib/python2.7/os.py", line 344, in execvp
  _execvpe(file, args) File "/home/user/userenv/lib/python2.7/os.py", line 380, in _execvpe func(fullname, *argrest)
  OSError: [Errno 2] No such file or directory`
  </code></pre>
  
  <p>But <code>airflow webserver -p 8051 -d</code> works</p>
  
  <p>Also I can't see <code>airflow-webserver.pid</code> file in <code>$AIRFLOW_HOME</code></p>
  
  <p>But in debug mode I can't run airflow worker</p>