Jupyter proxy in-depended from hub

[sverhoeven]

At the moment the proxy is restarted when JupyterHub is restarted, this causes users from loosing their connection and unable to work while the hub is down. In old versions of JupyterHub c.JupyterHub.cleanup_proxy = False to prevent stopping of proxy, but in current version this option has been replaced with different config. We should

• [ ] create a jupyter proxy systemd service that only gets restarted when hub version changes
• [x] or accept that connections will be momentary lost while provisioning.

In both cases configuration/documentation changes are needed.

[sverhoeven]

For SRC the provisioning only runs during first boot of VM, so instead of upgrading JH you create a new VM.

Issue no longer valid under SRC.