Allow external modifications to config file keys not managed by the Operator

[skriss]

Right now, the Operator offers a limited set of customizations for Contour. Some users might like to add additional settings directly to the Contour config map, to perform additional customizations for their environments. However, the Operator prevents this, because it will immediately remove any external customizations to the config map as part of its reconciliation.

One option would be to change the Operator's config map reconciliation to be more forgiving: it could ensure that the keys that it does manage are present and set to the right values, but not remove any additional settings that appear in the actual config map. I think this might be a more sensible approach, at least for now, to allow users more flexibility to customize their Operator-deployed Contours.