Default image tags to .Charts.AppVersion

Hey,

A lot of Helm charts out there fall back to the `appVersion` in `Chart.yaml` if no tag is given for the Docker image, so that you can update it dynamically using `helm package mychart --app-version 1.2.3`.

I took a look at `_helpers.tpl`, but it looks very intricate and no chance of getting a hold of `.Charts` inside the `define "kong.getRepoTag"`. For now, I am `sed`'ing the tag to my chart version before `helm package`. Could there be a nicer solution?

Cheers

Start in _helpers.tpl, especially the define "kong.getRepoTag" helper, and trace how image tags are currently selected. Verify how Helm exposes the chart appVersion and define the completed behavior as using it when no Docker image tag is provided, while preserving explicitly supplied tags.

Written by the indexing model from the issue text.