Allow templated values in initContainers field

[jeroen-vankelecom]

While you can add initContainers to the Kong deployment, it is a bit unfortunate that the value is static and cannot digest a templated value (e.g. use a template for the image).

Passing the initContainer values through the Helm template engine would give more flexibility to the chart end user.
You can find a random example on how other charts are managing this here.