WIP : Use certificateservice

[Ph0tonic]

Use CertificateService to retrieve certificate instead of relying on internal knowledge of CertificateService.

[Ph0tonic]

Note for myself:

The current implementation fails because when jupyterhub restarts, it does the following:

• Tries to reconnect to sessions and create an ARCSpawner
• When ARCSpawner tries to get certificates fro certificateservice, the service is not yet started, leading to a crash.

What could be done:

• Make certificateservice creation independent of jupyterhub pod, by using a custom Deploy
• But then, those services would not be managed by jupyterhub anymore leading to the unavailability of a JUPYTERHUB_API_TOKEN for the service
• We will have to check whether this is blocking or not for certificateservice, but I do not think so

Warning, we would have to check that the API is already available at the time of the creation of ARCSpawner to ensure that certificateservice can indeed contact the API to authenticate the request made by batchspawner.

[volodymyrss]

Note for myself:

The current implementation fails because when jupyterhub restarts, it does the following:

* Tries to reconnect to sessions and create an ARCSpawner 
* When ARCSpawner tries to get certificates fro certificateservice, the service is not yet started, leading to a crash.

Is this blocking (i.e. services will not start until all sessions reconnected by creating ARCSpawner) or a just race condition?