Add python-persistent helm chart.

[steveworley]

Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated
• PR title is ready for changelog and subsystem label(s) applied

Explain the details for making this change. What existing problem does the pull request solve?

Some python applications are currently deployed with the node-persistent service type to allow binding a persistent volume to the deployment. This introduces a python-persistent service type for semantic separation of deployments.

The main driver behind this is CKAN deployments as they currently rely on a PV. The node-persistent service type uses a TCP liveness probe rather than HTTP. This means that we cannot include Layer7 validations for CKAN to get Kubernetes to restart the pods.

The python liveness probe is configured for HTTP, so we can add additional listeners and application lookups to determine if a scaling event has moved the PG pod and have kubernetes restart the CKAN pod.

Closing issues

Closes #2988

[tobybellwood]

Thanks for this - it looks pretty good! Only pickup from me is that the cronjob isn't configured to have the volume mounts, so any cronjobs that run as separate containers won't have them. It should be easy enough to copy across from another service (e.g. node-persistent)

[steveworley]

Ah! Yeah - forgot to add the PV to the cron pod, will update the PR thanks!

[tobybellwood]

Tested locally by lablelling existing python docker-compose

    labels:
      lagoon.type: python-persistent
      lagoon.persistent: /files

Works as expected 🎉