Allow to configure Flower's url_prefix

[ecerulm]

When deploying with the Helm chart using an ingress for flower that has a path (ingress.flower.path) flower is currently unusable.
Flower by default assumes that it will be serving url starting on / and it must be told via url_prefix if it's actual url is not based directly on /.
Currently there is no way (that I know of) to pass url_prefix to the Flower deployment/pod.

This PR introduces a new helm chart value flower.url_prefix to allow for that.

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[ecerulm]

@mik-laj since you are reviewing #12619 , I guess it make sense if you review #12630 and #12634 as all of them are about getting ingresses to work properly.

[junnplus]

What's the progress for this pr? I also need flower.url_prefix for flower ui

[ashb]

There is already a config setting for this -- flower_url_prefix under the celery section -- we don't need a second method of configuring this do we?

[ashb]

Ah.

Instead of adding a new setting for this, we should

Suggested change

	{{- if .Values.flower.url_prefix }}
	- name: FLOWER_URL_PREFIX
	value: {{.Values.flower.url_prefix }}
	{{- end }}
	{{- include "custom_airflow_environment" . | indent 10 }}

Then the existing environment variable would work. Right now without any changes this can be set in the airflow.cfg config map.

[ecerulm]

Ok, I've dropped all other changes and just added this include.

[ecerulm]

There is already a config setting for this -- flower_url_prefix under the celery section -- we don't need a second method of configuring this do we?

@ashb, I don't understand. I can't find any flower_url_prefix in the project. I assume that you mean the existing ingress.flower.path.
The problem is that if you expose flower in a path that is not just / then flower will refuse to work. You need to tell flower that it's being server under another /path/* in order to do that I use the FLOWER_URL_PREFIX environment variable.

The ideal would be to generate the FLOWER_URL_PREFIX from ingress.flower.path but it's not trivial in the general case (ingress.flower.path as it's has the default pathType: ImplementationSpecific ) so I added as a new chart value.

[ecerulm]

Ok, nevermind. Yes there is flower_url_prefix that can be set with AIRFLOW__CELERY__FLOWER_URL_PREFIX