Related to #8036
We have had this behavior for a _long_ time now (since 0.7.0) but it
seems people are still periodically getting bit by it. This adds an
explicit error message that explains that this kind of override isn't
allowed anymore.
A JSON object will be decoded as a list with a single map value. This
will be properly coerced later, so let it through the initial config
semantic checks.
This is the first step in allowing overrides of map and list variables.
We convert Context.variables to map[string]interface{} from
map[string]string and fix up all the call sites.
Mitchell Hashimoto