The current implementation of the excluded_resources field on the MigrationController CR accepts a list of resources to exclude that actually overrides the default values, meaning that if you only set one new resource, the existing set is not carried forward. Our advice has been to copy the values of the default file: (https://github.com/konveyor/mig-operator/blob/master/roles/migrationcontroller/defaults/main.yml#L10-L24) but this is really not a good experience. Furthermore, our documentation does not reference this, either upstream (https://github.com/konveyor/mig-operator/blob/master/docs/usage/ExcludeResources.md) or downstream, AFAICT.

First, we should document how this field actually behaves and our expectations for its usage, including a caveat that this will override the (important) default set.

Secondly, let's consider an additional field that also accepts a set, and in the operator merge these both to produce the final set.

Spawned from: https://bugzilla.redhat.com/show_bug.cgi?id=2011861