Allow specifying Helm values for operator installations

[NickLarsenNZ]

When installing stacks/demos, the operator charts are installed with default Helm values.

It would be good to be able to

• override operator Helm values in stacks (eg: specify the ListenerClass presets for listener-operator)
• install/upgrade operators/release with custom Helm values (but be careful since the values are not consistent across all operators).

Considerations:

• How to make this look nice (because the command line could get rather long if having to specify each operator's overrides).
• Should we make it look under overrides/ (or similar) for ${listener-name}.yaml?
  • Should there be a flag to opt-in/out so that if the overrides/ (or similar) exists, it can be igored?
• Should we allow inlined values overrides in the stack definition, or just use the above?

Also, see issue about listener-operator (not only for stacks/demos): #356