Add event log

# JupyterHub Start with event log redirection; & cannot be passed in yaml files
jupyterhub_event_log:
  mountPath: /etc/jupyterhub/custom/event_command.sh
  stringData: |
    #! /bin/bash
    /usr/local/bin/jupyterhub --config /usr/local/etc/jupyterhub/jupyterhub_config.py --upgrade-db &>> /srv/jupyterhub/event.log
  mode: 0777

args: [ "/etc/jupyterhub/custom/event_command.sh" ]

requested review from @youngsu_kim

assigned to @youngsu_kim

mentioned in commit 72f92893

merged