fix default value of hub_url in config

[bitnik]

In gesiscss/persistent_binderhub#5 we found out that if someone installs BinderHub with auth enabled,
deployment fails with
Error: render error in "persistent_binderhub/charts/binderhub/templates/deployment.yaml": template: persistent_binderhub/charts/binderhub/templates/deployment.yaml:98:74: executing "persistent_binderhub/charts/binderhub/templates/deployment.yaml" at <"/">: invalid value; expected string

The error indicates

binderhub/helm-chart/binderhub/templates/deployment.yaml

Line 98 in 23ae206

value: {{ (print (.Values.config.BinderHub.hub_url | trimSuffix "/") "/hub/api/") }}

and it happens only if the deployment is not an upgrade from a standard BinderHub deployment (auth disabled), but it is an installation of BinderHub directly with authentication included. The reason is that on cloud-based environments BinderHub.hub_url is missing during the first round of the installation (https://binderhub.readthedocs.io/en/latest/zero-to-binderhub/setup-binderhub.html#install-binderhub), because it is provided after the first installation when JupyerterHub is ready (https://binderhub.readthedocs.io/en/latest/zero-to-binderhub/setup-binderhub.html#connect-binderhub-and-jupyterhub).

This PR fixes the default value of BinderHub.hub_url in helm config, so helm won't complain about it during the first installation and then user can set BinderHub.hub_url when it is available. It also fixes it binderhub_config.py, so it uses default value of hub_url "" instead of LazyConfigValue.

[bitnik]

Actually now I am thinking that instead of my first changes, it would be better to set the default value in values.yaml as

config:
  BinderHub:
    hub_url: ''

what do you think?

[bitnik]

@minrk I am not sure how to continue here. Could you do another review?

[consideRatio]

Actually now I am thinking that instead of my first changes, it would be better to set the default value in values.yaml as

config:
  BinderHub:
    hub_url: ''

what do you think?

I agree! That is what we do in z2jh as well I think, and it makes it clearer to the reader of the config than if the default value is different from the configured empty string etc.

@bitnik this LGTM, sorry you have had to experience such a delay on getting review help. I'm 100% committed to helping this get merged and could even take over if you want. Thanks for your work in BinderHub!!!!!! ❤️