helm: harmonise the treatment of reana_hostname

[tiborsimko]

Current behaviour

In the REANA Helm chart, we have reana_hostname commented out, and a somewhat inconsistent handling of reana_hostname's default value in case the admin does not configure it:

$ rg reana_hostname
helm/reana/templates/secrets.yaml
98:  {{- $cert := genSelfSignedCert (.Values.reana_hostname | default "localhost") nil nil 90 }}

helm/reana/templates/reana-workflow-controller.yaml
111:            value: {{ .Values.reana_hostname | default "" | quote  }}

helm/reana/templates/NOTES.txt
32:    $ firefox https://{{ default "localhost:30443" .Values.reana_hostname }}
39:    $ export REANA_SERVER_URL=https://{{ default "localhost:30443" .Values.reana_hostname }}

Analogously, in the REANA component source code base, the environment variable REANA_HOSTNAME reading is handled a bit differently in different components:

$ rg REANA_HOSTNAME | grep 'os\.'
reana-server/reana_server/config.py:REANA_HOSTNAME = os.getenv("REANA_HOSTNAME")
reana-workflow-controller/reana_workflow_controller/config.py:REANA_HOSTNAME = os.getenv("REANA_HOSTNAME", "CHANGE_ME")

This causes certain URLs to not look pretty, for example notice the CHANGE_ME in the Dask dashboard URL:

$ kubectl exec -i -t deployment/reana-db -- psql -U reana
psql (14.10 (Debian 14.10-1.pgdg120+1))
Type "help" for help.

reana=# select * from __reana.service;
                 id_                  |         name          |                                   uri                                   | status  | owner_id | type_
--------------------------------------+-----------------------+-------------------------------------------------------------------------+---------+----------+-------
 6eb43922-3c90-44b3-b21c-d555a4f8725f | dask-service-f24f0a14 | https://CHANGE_ME/f24f0a14-b59c-40eb-8104-53d77286ce5a/dashboard/status | running |          | dask
(1 row)

Expected behaviour

It would be good to harmonise the treatment of reana_hostname across the code base. For example, we could require it, and fill some convenient default:

• e.g. switch everything to use localhost as default, provided that this value would be OK for various local development deployment scenarios (such as Docker Desktop VM on macOS).

• e.g. switch everything to use something else (say 127.0.0.1) as default, if that is better for various deployment scenarios;

• but we should not have other default values such as CHANGE_ME across components.