[Fixes #152] Feature: add image pull policy to all container

[mwallschlaeger]

Description

PR makes ImagePullPolicy configurable for all, containers defined within this chart. It further add configuration for init containers and configures init containers to use a fixed version.

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• Bug fix
• New feature
• Documentation update
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closes #152

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[Zalfsten]

A question/comment concerning imagePullPolicy: best practice is to use a digest hash to specify the image (instead of 'latest' for a version number) -- at least for productive purposes. In this case I would say, the imagePullPolicy does not matter any more? Either is image with the correct hash is available, then nothing needs to be downloaded, or it's not and thus must be downloaded...

[mwallschlaeger]

This is usually correct. But e.g. in GeoNode context it happens that geosolutions provides a new version of an image but using the same image tag. Generally it is a nice to have for users of this chart to set this policy on their on. Further this PR adds fixed versions for init containers and adds possibility to change the init container images and tags.