[bitnami/kube-prometheus] Add possibility to pull images without giving registry name

[EtienneBarbier]

Description of the change

Add possibility to pull images prometheusConfigReload Image without giving specific registry in Values.yaml file but by setting default registry on host. See #5582

Benefits

We can use non default prometheusConfigReload if we omit .Values.operator.prometheusConfigReloader.image.registry

Possible drawbacks

We cannot use default behavior for prometheusConfigReload by omitting .Values.operator.prometheusConfigReloader.image.registry, we need to omit imageName or imageTag.

Applicable issues

• fixes [bitnami/kube-prometheus] Prometheus Operator Reloader image weird behavior #5584

Additional information

None

Checklist

• Chart version bumped in Chart.yaml according to semver.
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [bitnami/chart])

⚠️ Keep in mind that if you want to make changes to the kubeapps chart, please implement them in the kubeapps repository. This is only a synchronized mirror.

[alemorcuq]

LGTM, thanks!