This helps in logging up the requests made by ipython_notebook service

[ kirr: To be clear - until log-level is set to DEBUG, IPython notebook does
  not log HTTP requests, and since logging of HTTP requests is considered normal
  for most of our services (Zope, Apache, etc), it makes sense to enable such
  functionality for notebook too.

  There is not much additional noise produced by --log-level=DEBUG - in
  practice ipython only prints what config files it uses on startup, so this
  should be ok to go. ]

/reviewed-by @kirr, @jerome  (on nexedi/slapos!33)