Support lookup functions in the helm template calls

[voor]

Describe the problem/challenge you have
More charts are starting to implement the helm lookup function which will check existing secrets for certain values and populate them -- this provides a convenience and form of error checking in certain charts to make sure values are populated and correct. This creates problems for kapp which expects the helm template calls to not interact with existing cluster data.

Describe the solution you'd like
Allow the helm template call to use the service account that exists for the App CR to potentially get some data off the cluster. This would require adding --dry-run=server to the helm template command and providing a kubeconfig that would be accessible to the client.

Anything else you would like to add:
I don't like this, but helm chart authors are starting to implement this function in a way that breaks certain functionality if it's not used.

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[voor]

Example: the Bitnami Redis chart won't let you use an existing secret anymore as a values, since if you want to use an existing secret it will lookup the value in that secret and then use it in the rendering instead of just letting you put in the reference to the secret. See here: Bitnami Redis chart template