Add config option to define behaviour when no groups are set

[vpavlin]

Related Issues and Dependencies

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This introduces a breaking change

• [ ] Yes
• [x] No

This Pull Request implements

This PR adds an environment variable JUPYTERHUB_GROUPS_DEFAULT_DENIED which defines if users should be allowed to use JupyterHub if no groups are defined.

By default all users with OpenShift login are allowed

By setting this env var to true you make sure all users are by default denied access, unless the groups are configured and the users belong to given groups

Description

[anishasthana]

While testing, if we set JUPYTERHUB_GROUPS_DEFAULT_DENIED to "true" and didn't have any groups created, JupyterHub gets stuck in a crashloop. Is this behaviour expected?

[harshad16]

/retest