implement chart upgrade script + bump kong chart to 1.8.0

[mflendrich]

This PR

• implements a script which automates chart upgrades in a simple and predictable way
• upgrades the kong helm chart to 1.8.0

The scripted way of updating the helm chart makes it possible to add a validation on CI that the chart does is an exact copy of the upstream kong chart without local changes.

[mflendrich]

The change has been tested to the extent currently performed by CI.

[rainest]

Generally approved content-wise, but sticking a DO NOT MERGE on this for now.

In light of #24 I think we should look at merging https://github.com/Kong/charts/pull/179 and and https://github.com/Kong/charts/pull/175, cutting a 1.9 release from those, and then sticking that over top of 1.8 in this PR (or a second, though I don't see any harm in keeping this one open and updating over top of it, since the script wouldn't need any updates, just the files we copy from the chart).

Barring opposition to the busybox replacement, that puts us in a better place to handle #24, if not entirely then at least without having to integrate chart changes on top of the manifest/OLM changes needed for the certified release.

[mflendrich]

Per offline discussion, removing DO NOT MERGE because Kong/charts#179 and Kong/charts#175 will go to 1.9.0 (and therefore don't affect 1.8.0), and we chose not to skip an operator release using the 1.8.0 chart (to go straight to 1.9.0).