User defined role overrides are ignored by compute_files()

[razvan]

Description

Given a fictional CRD with role level configuration overrides like this:

brokers:
  configOverrides:
    server.properties:
      auto.create.topics.enable: "false"
  roleGroups:
    default:
      replicas: 1

and an implementation of Configuration::compute_files() that sets auto.create.topics.enable to true,
then the user defined override is ignored and the value of the property will be true.

Note that this doesn't happen with configuration overrides at group level!

The expectation of overrides is, that whatever the user has set , wins over everything else.

A test that demonstrates the bug is here: #338